1Parse::RecDescent(3) User Contributed Perl Documentation Parse::RecDescent(3)
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6 Parse::RecDescent - Generate Recursive-Descent Parsers
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9 This document describes version 1.967015 of Parse::RecDescent released
10 April 4th, 2017.
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13 use Parse::RecDescent;
14 # Generate a parser from the specification in $grammar:
16 $parser = new Parse::RecDescent ($grammar);
18 # Generate a parser from the specification in $othergrammar
20 $anotherparser = new Parse::RecDescent ($othergrammar);
22 # Parse $text using rule 'startrule' (which must be
25 # defined in $grammar):
26 $parser->startrule($text);
28 # Parse $text using rule 'otherrule' (which must also
31 # be defined in $grammar):
32 $parser->otherrule($text);
34 # Change the universal token prefix pattern
37 # before building a grammar
38 # (the default is: '\s*'):
39 $Parse::RecDescent::skip = '[ \t]+';
41 # Replace productions of existing rules (or create new ones)
44 # with the productions defined in $newgrammar:
45 $parser->Replace($newgrammar);
47 # Extend existing rules (or create new ones)
50 # by adding extra productions defined in $moregrammar:
51 $parser->Extend($moregrammar);
53 # Global flags (useful as command line arguments under -s):
56 $::RD_ERRORS # unless undefined, report fatal errors
58 $::RD_WARN # unless undefined, also report non-fatal problems
59 $::RD_HINT # if defined, also suggestion remedies
60 $::RD_TRACE # if defined, also trace parsers' behaviour
61 $::RD_AUTOSTUB # if defined, generates "stubs" for undefined rules
62 $::RD_AUTOACTION # if defined, appends specified action to productions
63
65 Overview
66 Parse::RecDescent incrementally generates top-down recursive-descent
67 text parsers from simple yacc-like grammar specifications. It provides:
68 · Regular expressions or literal strings as terminals (tokens),
70 · Multiple (non-contiguous) productions for any rule,
72 · Repeated and optional subrules within productions,
74 · Full access to Perl within actions specified as part of the
76 grammar,
77 · Simple automated error reporting during parser generation and
79 parsing,
80 · The ability to commit to, uncommit to, or reject particular
82 productions during a parse,
83 · The ability to pass data up and down the parse tree ("down" via
85 subrule argument lists, "up" via subrule return values)
86 · Incremental extension of the parsing grammar (even during a parse),
88 · Precompilation of parser objects,
90 · User-definable reduce-reduce conflict resolution via "scoring" of
92 matching productions.
93 Using "Parse::RecDescent"
95 Parser objects are created by calling "Parse::RecDescent::new", passing
96 in a grammar specification (see the following subsections). If the
97 grammar is correct, "new" returns a blessed reference which can then be
98 used to initiate parsing through any rule specified in the original
99 grammar. A typical sequence looks like this:
100 $grammar = q {
102 # GRAMMAR SPECIFICATION HERE
103 };
104 $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n";
106 # acquire $text
108 defined $parser->startrule($text) or print "Bad text!\n";
110 The rule through which parsing is initiated must be explicitly defined
112 in the grammar (i.e. for the above example, the grammar must include a
113 rule of the form: "startrule: <subrules>".
114 If the starting rule succeeds, its value (see below) is returned.
116 Failure to generate the original parser or failure to match a text is
117 indicated by returning "undef". Note that it's easy to set up grammars
118 that can succeed, but which return a value of 0, "0", or "". So don't
119 be tempted to write:
120 $parser->startrule($text) or print "Bad text!\n";
122 Normally, the parser has no effect on the original text. So in the
124 previous example the value of $text would be unchanged after having
125 been parsed.
126 If, however, the text to be matched is passed by reference:
128 $parser->startrule(\$text)
130 then any text which was consumed during the match will be removed from
132 the start of $text.
133 Rules
135 In the grammar from which the parser is built, rules are specified by
136 giving an identifier (which must satisfy /[A-Za-z]\w*/), followed by a
137 colon on the same line, followed by one or more productions, separated
138 by single vertical bars. The layout of the productions is entirely
139 free-format:
140 rule1: production1
142 | production2 |
143 production3 | production4
144 At any point in the grammar previously defined rules may be extended
146 with additional productions. This is achieved by redeclaring the rule
147 with the new productions. Thus:
148 rule1: a | b | c
150 rule2: d | e | f
151 rule1: g | h
152 is exactly equivalent to:
154 rule1: a | b | c | g | h
156 rule2: d | e | f
157 Each production in a rule consists of zero or more items, each of which
159 may be either: the name of another rule to be matched (a "subrule"), a
160 pattern or string literal to be matched directly (a "token"), a block
161 of Perl code to be executed (an "action"), a special instruction to the
162 parser (a "directive"), or a standard Perl comment (which is ignored).
163 A rule matches a text if one of its productions matches. A production
165 matches if each of its items match consecutive substrings of the text.
166 The productions of a rule being matched are tried in the same order
167 that they appear in the original grammar, and the first matching
168 production terminates the match attempt (successfully). If all
169 productions are tried and none matches, the match attempt fails.
170 Note that this behaviour is quite different from the "prefer the longer
172 match" behaviour of yacc. For example, if yacc were parsing the rule:
173 seq : 'A' 'B'
175 | 'A' 'B' 'C'
176 upon matching "AB" it would look ahead to see if a 'C' is next and, if
178 so, will match the second production in preference to the first. In
179 other words, yacc effectively tries all the productions of a rule
180 breadth-first in parallel, and selects the "best" match, where "best"
181 means longest (note that this is a gross simplification of the true
182 behaviour of yacc but it will do for our purposes).
183 In contrast, "Parse::RecDescent" tries each production depth-first in
185 sequence, and selects the "best" match, where "best" means first. This
186 is the fundamental difference between "bottom-up" and "recursive
187 descent" parsing.
188 Each successfully matched item in a production is assigned a value,
190 which can be accessed in subsequent actions within the same production
191 (or, in some cases, as the return value of a successful subrule call).
192 Unsuccessful items don't have an associated value, since the failure of
193 an item causes the entire surrounding production to immediately fail.
194 The following sections describe the various types of items and their
195 success values.
196 Subrules
198 A subrule which appears in a production is an instruction to the parser
199 to attempt to match the named rule at that point in the text being
200 parsed. If the named subrule is not defined when requested the
201 production containing it immediately fails (unless it was "autostubbed"
202 - see Autostubbing).
203 A rule may (recursively) call itself as a subrule, but not as the left-
205 most item in any of its productions (since such recursions are usually
206 non-terminating).
207 The value associated with a subrule is the value associated with its
209 $return variable (see "Actions" below), or with the last successfully
210 matched item in the subrule match.
211 Subrules may also be specified with a trailing repetition specifier,
213 indicating that they are to be (greedily) matched the specified number
214 of times. The available specifiers are:
215 subrule(?) # Match one-or-zero times
217 subrule(s) # Match one-or-more times
218 subrule(s?) # Match zero-or-more times
219 subrule(N) # Match exactly N times for integer N > 0
220 subrule(N..M) # Match between N and M times
221 subrule(..M) # Match between 1 and M times
222 subrule(N..) # Match at least N times
223 Repeated subrules keep matching until either the subrule fails to
225 match, or it has matched the minimal number of times but fails to
226 consume any of the parsed text (this second condition prevents the
227 subrule matching forever in some cases).
228 Since a repeated subrule may match many instances of the subrule
230 itself, the value associated with it is not a simple scalar, but rather
231 a reference to a list of scalars, each of which is the value associated
232 with one of the individual subrule matches. In other words in the rule:
233 program: statement(s)
235 the value associated with the repeated subrule "statement(s)" is a
237 reference to an array containing the values matched by each call to the
238 individual subrule "statement".
239 Repetition modifiers may include a separator pattern:
241 program: statement(s /;/)
243 specifying some sequence of characters to be skipped between each
245 repetition. This is really just a shorthand for the <leftop:...>
246 directive (see below).
247 Tokens
249 If a quote-delimited string or a Perl regex appears in a production,
250 the parser attempts to match that string or pattern at that point in
251 the text. For example:
252 typedef: "typedef" typename identifier ';'
254 identifier: /[A-Za-z_][A-Za-z0-9_]*/
256 As in regular Perl, a single quoted string is uninterpolated, whilst a
258 double-quoted string or a pattern is interpolated (at the time of
259 matching, not when the parser is constructed). Hence, it is possible to
260 define rules in which tokens can be set at run-time:
261 typedef: "$::typedefkeyword" typename identifier ';'
263 identifier: /$::identpat/
265 Note that, since each rule is implemented inside a special namespace
267 belonging to its parser, it is necessary to explicitly quantify
268 variables from the main package.
269 Regex tokens can be specified using just slashes as delimiters or with
271 the explicit "m<delimiter>......<delimiter>" syntax:
272 typedef: "typedef" typename identifier ';'
274 typename: /[A-Za-z_][A-Za-z0-9_]*/
276 identifier: m{[A-Za-z_][A-Za-z0-9_]*}
278 A regex of either type can also have any valid trailing parameter(s)
280 (that is, any of [cgimsox]):
281 typedef: "typedef" typename identifier ';'
283 identifier: / [a-z_] # LEADING ALPHA OR UNDERSCORE
285 [a-z0-9_]* # THEN DIGITS ALSO ALLOWED
286 /ix # CASE/SPACE/COMMENT INSENSITIVE
287 The value associated with any successfully matched token is a string
289 containing the actual text which was matched by the token.
290 It is important to remember that, since each grammar is specified in a
292 Perl string, all instances of the universal escape character '\' within
293 a grammar must be "doubled", so that they interpolate to single '\'s
294 when the string is compiled. For example, to use the grammar:
295 word: /\S+/ | backslash
297 line: prefix word(s) "\n"
298 backslash: '\\'
299 the following code is required:
301 $parser = new Parse::RecDescent (q{
303 word: /\\S+/ | backslash
305 line: prefix word(s) "\\n"
306 backslash: '\\\\'
307 });
309 Anonymous subrules
311 Parentheses introduce a nested scope that is very like a call to an
312 anonymous subrule. Hence they are useful for "in-lining" subroutine
313 calls, and other kinds of grouping behaviour. For example, instead of:
314 word: /\S+/ | backslash
316 line: prefix word(s) "\n"
317 you could write:
319 line: prefix ( /\S+/ | backslash )(s) "\n"
321 and get exactly the same effects.
323 Parentheses are also use for collecting unrepeated alternations within
325 a single production.
326 secret_identity: "Mr" ("Incredible"|"Fantastic"|"Sheen") ", Esq."
328 Terminal Separators
330 For the purpose of matching, each terminal in a production is
331 considered to be preceded by a "prefix" - a pattern which must be
332 matched before a token match is attempted. By default, the prefix is
333 optional whitespace (which always matches, at least trivially), but
334 this default may be reset in any production.
335 The variable $Parse::RecDescent::skip stores the universal prefix,
337 which is the default for all terminal matches in all parsers built with
338 "Parse::RecDescent".
339 If you want to change the universal prefix using
341 $Parse::RecDescent::skip, be careful to set it before creating the
342 grammar object, because it is applied statically (when a grammar is
343 built) rather than dynamically (when the grammar is used).
344 Alternatively you can provide a global "<skip:...>" directive in your
345 grammar before any rules (described later).
346 The prefix for an individual production can be altered by using the
348 "<skip:...>" directive (described later). Setting this directive in
349 the top-level rule is an alternative approach to setting
350 $Parse::RecDescent::skip before creating the object, but in this case
351 you don't get the intended skipping behaviour if you directly invoke
352 methods different from the top-level rule.
353 Actions
355 An action is a block of Perl code which is to be executed (as the block
356 of a "do" statement) when the parser reaches that point in a
357 production. The action executes within a special namespace belonging to
358 the active parser, so care must be taken in correctly qualifying
359 variable names (see also "Start-up Actions" below).
360 The action is considered to succeed if the final value of the block is
362 defined (that is, if the implied "do" statement evaluates to a defined
363 value - even one which would be treated as "false"). Note that the
364 value associated with a successful action is also the final value in
365 the block.
366 An action will fail if its last evaluated value is "undef". This is
368 surprisingly easy to accomplish by accident. For instance, here's an
369 infuriating case of an action that makes its production fail, but only
370 when debugging isn't activated:
371 description: name rank serial_number
373 { print "Got $item[2] $item[1] ($item[3])\n"
374 if $::debugging
375 }
376 If $debugging is false, no statement in the block is executed, so the
378 final value is "undef", and the entire production fails. The solution
379 is:
380 description: name rank serial_number
382 { print "Got $item[2] $item[1] ($item[3])\n"
383 if $::debugging;
384 1;
385 }
386 Within an action, a number of useful parse-time variables are available
388 in the special parser namespace (there are other variables also
389 accessible, but meddling with them will probably just break your
390 parser. As a general rule, if you avoid referring to unqualified
391 variables - especially those starting with an underscore - inside an
392 action, things should be okay):
393 @item and %item
395 The array slice @item[1..$#item] stores the value associated with
396 each item (that is, each subrule, token, or action) in the current
397 production. The analogy is to $1, $2, etc. in a yacc grammar. Note
398 that, for obvious reasons, @item only contains the values of items
399 before the current point in the production.
400 The first element ($item[0]) stores the name of the current rule
402 being matched.
403 @item is a standard Perl array, so it can also be indexed with
405 negative numbers, representing the number of items back from the
406 current position in the parse:
407 stuff: /various/ bits 'and' pieces "then" data 'end'
409 { print $item[-2] } # PRINTS data
410 # (EASIER THAN: $item[6])
411 The %item hash complements the <@item> array, providing named
413 access to the same item values:
414 stuff: /various/ bits 'and' pieces "then" data 'end'
416 { print $item{data} # PRINTS data
417 # (EVEN EASIER THAN USING @item)
418 The results of named subrules are stored in the hash under each
420 subrule's name (including the repetition specifier, if any), whilst
421 all other items are stored under a "named positional" key that
422 indicates their ordinal position within their item type:
423 __STRINGn__, __PATTERNn__, __DIRECTIVEn__, __ACTIONn__:
424 stuff: /various/ bits 'and' pieces "then" data 'end' { save }
426 { print $item{__PATTERN1__}, # PRINTS 'various'
427 $item{__STRING2__}, # PRINTS 'then'
428 $item{__ACTION1__}, # PRINTS RETURN
429 # VALUE OF save
430 }
431 If you want proper named access to patterns or literals, you need
433 to turn them into separate rules:
434 stuff: various bits 'and' pieces "then" data 'end'
436 { print $item{various} # PRINTS various
437 }
438 various: /various/
440 The special entry $item{__RULE__} stores the name of the current
442 rule (i.e. the same value as $item[0].
443 The advantage of using %item, instead of @items is that it removes
445 the need to track items positions that may change as a grammar
446 evolves. For example, adding an interim "<skip>" directive of
447 action can silently ruin a trailing action, by moving an @item
448 element "down" the array one place. In contrast, the named entry of
449 %item is unaffected by such an insertion.
450 A limitation of the %item hash is that it only records the last
452 value of a particular subrule. For example:
453 range: '(' number '..' number )'
455 { $return = $item{number} }
456 will return only the value corresponding to the second match of the
458 "number" subrule. In other words, successive calls to a subrule
459 overwrite the corresponding entry in %item. Once again, the
460 solution is to rename each subrule in its own rule:
461 range: '(' from_num '..' to_num ')'
463 { $return = $item{from_num} }
464 from_num: number
466 to_num: number
467 @arg and %arg
469 The array @arg and the hash %arg store any arguments passed to the
470 rule from some other rule (see "Subrule argument lists"). Changes
471 to the elements of either variable do not propagate back to the
472 calling rule (data can be passed back from a subrule via the
473 $return variable - see next item).
474 $return
476 If a value is assigned to $return within an action, that value is
477 returned if the production containing the action eventually matches
478 successfully. Note that setting $return doesn't cause the current
479 production to succeed. It merely tells it what to return if it does
480 succeed. Hence $return is analogous to $$ in a yacc grammar.
481 If $return is not assigned within a production, the value of the
483 last component of the production (namely: $item[$#item]) is
484 returned if the production succeeds.
485 $commit
487 The current state of commitment to the current production (see
488 "Directives" below).
489 $skip
491 The current terminal prefix (see "Directives" below).
492 $text
494 The remaining (unparsed) text. Changes to $text do not propagate
495 out of unsuccessful productions, but do survive successful
496 productions. Hence it is possible to dynamically alter the text
497 being parsed - for example, to provide a "#include"-like facility:
498 hash_include: '#include' filename
500 { $text = ::loadfile($item[2]) . $text }
501 filename: '<' /[a-z0-9._-]+/i '>' { $return = $item[2] }
503 | '"' /[a-z0-9._-]+/i '"' { $return = $item[2] }
504 $thisline and $prevline
506 $thisline stores the current line number within the current parse
507 (starting from 1). $prevline stores the line number for the last
508 character which was already successfully parsed (this will be
509 different from $thisline at the end of each line).
510 For efficiency, $thisline and $prevline are actually tied hashes,
512 and only recompute the required line number when the variable's
513 value is used.
514 Assignment to $thisline adjusts the line number calculator, so that
516 it believes that the current line number is the value being
517 assigned. Note that this adjustment will be reflected in all
518 subsequent line numbers calculations.
519 Modifying the value of the variable $text (as in the previous
521 "hash_include" example, for instance) will confuse the line
522 counting mechanism. To prevent this, you should call
523 "Parse::RecDescent::LineCounter::resync($thisline)" immediately
524 after any assignment to the variable $text (or, at least, before
525 the next attempt to use $thisline).
526 Note that if a production fails after assigning to or resync'ing
528 $thisline, the parser's line counter mechanism will usually be
529 corrupted.
530 Also see the entry for @itempos.
532 The line number can be set to values other than 1, by calling the
534 start rule with a second argument. For example:
535 $parser = new Parse::RecDescent ($grammar);
537 $parser->input($text, 10); # START LINE NUMBERS AT 10
539 $thiscolumn and $prevcolumn
541 $thiscolumn stores the current column number within the current
542 line being parsed (starting from 1). $prevcolumn stores the column
543 number of the last character which was actually successfully
544 parsed. Usually "$prevcolumn == $thiscolumn-1", but not at the end
545 of lines.
546 For efficiency, $thiscolumn and $prevcolumn are actually tied
548 hashes, and only recompute the required column number when the
549 variable's value is used.
550 Assignment to $thiscolumn or $prevcolumn is a fatal error.
552 Modifying the value of the variable $text (as in the previous
554 "hash_include" example, for instance) may confuse the column
555 counting mechanism.
556 Note that $thiscolumn reports the column number before any
558 whitespace that might be skipped before reading a token. Hence if
559 you wish to know where a token started (and ended) use something
560 like this:
561 rule: token1 token2 startcol token3 endcol token4
563 { print "token3: columns $item[3] to $item[5]"; }
564 startcol: '' { $thiscolumn } # NEED THE '' TO STEP PAST TOKEN SEP
566 endcol: { $prevcolumn }
567 Also see the entry for @itempos.
569 $thisoffset and $prevoffset
571 $thisoffset stores the offset of the current parsing position
572 within the complete text being parsed (starting from 0).
573 $prevoffset stores the offset of the last character which was
574 actually successfully parsed. In all cases "$prevoffset ==
575 $thisoffset-1".
576 For efficiency, $thisoffset and $prevoffset are actually tied
578 hashes, and only recompute the required offset when the variable's
579 value is used.
580 Assignment to $thisoffset or <$prevoffset> is a fatal error.
582 Modifying the value of the variable $text will not affect the
584 offset counting mechanism.
585 Also see the entry for @itempos.
587 @itempos
589 The array @itempos stores a hash reference corresponding to each
590 element of @item. The elements of the hash provide the following:
591 $itempos[$n]{offset}{from} # VALUE OF $thisoffset BEFORE $item[$n]
593 $itempos[$n]{offset}{to} # VALUE OF $prevoffset AFTER $item[$n]
594 $itempos[$n]{line}{from} # VALUE OF $thisline BEFORE $item[$n]
595 $itempos[$n]{line}{to} # VALUE OF $prevline AFTER $item[$n]
596 $itempos[$n]{column}{from} # VALUE OF $thiscolumn BEFORE $item[$n]
597 $itempos[$n]{column}{to} # VALUE OF $prevcolumn AFTER $item[$n]
598 Note that the various "$itempos[$n]...{from}" values record the
600 appropriate value after any token prefix has been skipped.
601 Hence, instead of the somewhat tedious and error-prone:
603 rule: startcol token1 endcol
605 startcol token2 endcol
606 startcol token3 endcol
607 { print "token1: columns $item[1]
608 to $item[3]
609 token2: columns $item[4]
610 to $item[6]
611 token3: columns $item[7]
612 to $item[9]" }
613 startcol: '' { $thiscolumn } # NEED THE '' TO STEP PAST TOKEN SEP
615 endcol: { $prevcolumn }
616 it is possible to write:
618 rule: token1 token2 token3
620 { print "token1: columns $itempos[1]{column}{from}
621 to $itempos[1]{column}{to}
622 token2: columns $itempos[2]{column}{from}
623 to $itempos[2]{column}{to}
624 token3: columns $itempos[3]{column}{from}
625 to $itempos[3]{column}{to}" }
626 Note however that (in the current implementation) the use of
628 @itempos anywhere in a grammar implies that item positioning
629 information is collected everywhere during the parse. Depending on
630 the grammar and the size of the text to be parsed, this may be
631 prohibitively expensive and the explicit use of $thisline,
632 $thiscolumn, etc. may be a better choice.
633 $thisparser
635 A reference to the "Parse::RecDescent" object through which parsing
636 was initiated.
637 The value of $thisparser propagates down the subrules of a parse
639 but not back up. Hence, you can invoke subrules from another parser
640 for the scope of the current rule as follows:
641 rule: subrule1 subrule2
643 | { $thisparser = $::otherparser } <reject>
644 | subrule3 subrule4
645 | subrule5
646 The result is that the production calls "subrule1" and "subrule2"
648 of the current parser, and the remaining productions call the named
649 subrules from $::otherparser. Note, however that "Bad Things" will
650 happen if "::otherparser" isn't a blessed reference and/or doesn't
651 have methods with the same names as the required subrules!
652 $thisrule
654 A reference to the "Parse::RecDescent::Rule" object corresponding
655 to the rule currently being matched.
656 $thisprod
658 A reference to the "Parse::RecDescent::Production" object
659 corresponding to the production currently being matched.
660 $score and $score_return
662 $score stores the best production score to date, as specified by an
663 earlier "<score:...>" directive. $score_return stores the
664 corresponding return value for the successful production.
665 See "Scored productions".
667 Warning: the parser relies on the information in the various "this..."
669 objects in some non-obvious ways. Tinkering with the other members of
670 these objects will probably cause Bad Things to happen, unless you
671 really know what you're doing. The only exception to this advice is
672 that the use of "$this...->{local}" is always safe.
673 Start-up Actions
675 Any actions which appear before the first rule definition in a grammar
676 are treated as "start-up" actions. Each such action is stripped of its
677 outermost brackets and then evaluated (in the parser's special
678 namespace) just before the rules of the grammar are first compiled.
679 The main use of start-up actions is to declare local variables within
681 the parser's special namespace:
682 { my $lastitem = '???'; }
684 list: item(s) { $return = $lastitem }
686 item: book { $lastitem = 'book'; }
688 bell { $lastitem = 'bell'; }
689 candle { $lastitem = 'candle'; }
690 but start-up actions can be used to execute any valid Perl code within
692 a parser's special namespace.
693 Start-up actions can appear within a grammar extension or replacement
695 (that is, a partial grammar installed via "Parse::RecDescent::Extend()"
696 or "Parse::RecDescent::Replace()" - see "Incremental Parsing"), and
697 will be executed before the new grammar is installed. Note, however,
698 that a particular start-up action is only ever executed once.
699 Autoactions
701 It is sometimes desirable to be able to specify a default action to be
702 taken at the end of every production (for example, in order to easily
703 build a parse tree). If the variable $::RD_AUTOACTION is defined when
704 "Parse::RecDescent::new()" is called, the contents of that variable are
705 treated as a specification of an action which is to appended to each
706 production in the corresponding grammar.
707 Alternatively, you can hard-code the autoaction within a grammar, using
709 the "<autoaction:...>" directive.
710 So, for example, to construct a simple parse tree you could write:
712 $::RD_AUTOACTION = q { [@item] };
714 parser = Parse::RecDescent->new(q{
716 expression: and_expr '||' expression | and_expr
717 and_expr: not_expr '&&' and_expr | not_expr
718 not_expr: '!' brack_expr | brack_expr
719 brack_expr: '(' expression ')' | identifier
720 identifier: /[a-z]+/i
721 });
722 or:
724 parser = Parse::RecDescent->new(q{
726 <autoaction: { [@item] } >
727 expression: and_expr '||' expression | and_expr
729 and_expr: not_expr '&&' and_expr | not_expr
730 not_expr: '!' brack_expr | brack_expr
731 brack_expr: '(' expression ')' | identifier
732 identifier: /[a-z]+/i
733 });
734 Either of these is equivalent to:
736 parser = new Parse::RecDescent (q{
738 expression: and_expr '||' expression
739 { [@item] }
740 | and_expr
741 { [@item] }
742 and_expr: not_expr '&&' and_expr
744 { [@item] }
745 | not_expr
746 { [@item] }
747 not_expr: '!' brack_expr
749 { [@item] }
750 | brack_expr
751 { [@item] }
752 brack_expr: '(' expression ')'
754 { [@item] }
755 | identifier
756 { [@item] }
757 identifier: /[a-z]+/i
759 { [@item] }
760 });
761 Alternatively, we could take an object-oriented approach, use different
763 classes for each node (and also eliminating redundant intermediate
764 nodes):
765 $::RD_AUTOACTION = q
767 { $#item==1 ? $item[1] : "$item[0]_node"->new(@item[1..$#item]) };
768 parser = Parse::RecDescent->new(q{
770 expression: and_expr '||' expression | and_expr
771 and_expr: not_expr '&&' and_expr | not_expr
772 not_expr: '!' brack_expr | brack_expr
773 brack_expr: '(' expression ')' | identifier
774 identifier: /[a-z]+/i
775 });
776 or:
778 parser = Parse::RecDescent->new(q{
780 <autoaction:
781 $#item==1 ? $item[1] : "$item[0]_node"->new(@item[1..$#item])
782 >
783 expression: and_expr '||' expression | and_expr
785 and_expr: not_expr '&&' and_expr | not_expr
786 not_expr: '!' brack_expr | brack_expr
787 brack_expr: '(' expression ')' | identifier
788 identifier: /[a-z]+/i
789 });
790 which are equivalent to:
792 parser = Parse::RecDescent->new(q{
794 expression: and_expr '||' expression
795 { "expression_node"->new(@item[1..3]) }
796 | and_expr
797 and_expr: not_expr '&&' and_expr
799 { "and_expr_node"->new(@item[1..3]) }
800 | not_expr
801 not_expr: '!' brack_expr
803 { "not_expr_node"->new(@item[1..2]) }
804 | brack_expr
805 brack_expr: '(' expression ')'
807 { "brack_expr_node"->new(@item[1..3]) }
808 | identifier
809 identifier: /[a-z]+/i
811 { "identifer_node"->new(@item[1]) }
812 });
813 Note that, if a production already ends in an action, no autoaction is
815 appended to it. For example, in this version:
816 $::RD_AUTOACTION = q
818 { $#item==1 ? $item[1] : "$item[0]_node"->new(@item[1..$#item]) };
819 parser = Parse::RecDescent->new(q{
821 expression: and_expr '&&' expression | and_expr
822 and_expr: not_expr '&&' and_expr | not_expr
823 not_expr: '!' brack_expr | brack_expr
824 brack_expr: '(' expression ')' | identifier
825 identifier: /[a-z]+/i
826 { 'terminal_node'->new($item[1]) }
827 });
828 each "identifier" match produces a "terminal_node" object, not an
830 "identifier_node" object.
831 A level 1 warning is issued each time an "autoaction" is added to some
833 production.
834 Autotrees
836 A commonly needed autoaction is one that builds a parse-tree. It is
837 moderately tricky to set up such an action (which must treat terminals
838 differently from non-terminals), so Parse::RecDescent simplifies the
839 process by providing the "<autotree>" directive.
840 If this directive appears at the start of grammar, it causes
842 Parse::RecDescent to insert autoactions at the end of any rule except
843 those which already end in an action. The action inserted depends on
844 whether the production is an intermediate rule (two or more items), or
845 a terminal of the grammar (i.e. a single pattern or string item).
846 So, for example, the following grammar:
848 <autotree>
850 file : command(s)
852 command : get | set | vet
853 get : 'get' ident ';'
854 set : 'set' ident 'to' value ';'
855 vet : 'check' ident 'is' value ';'
856 ident : /\w+/
857 value : /\d+/
858 is equivalent to:
860 file : command(s) { bless \%item, $item[0] }
862 command : get { bless \%item, $item[0] }
863 | set { bless \%item, $item[0] }
864 | vet { bless \%item, $item[0] }
865 get : 'get' ident ';' { bless \%item, $item[0] }
866 set : 'set' ident 'to' value ';' { bless \%item, $item[0] }
867 vet : 'check' ident 'is' value ';' { bless \%item, $item[0] }
868 ident : /\w+/ { bless {__VALUE__=>$item[1]}, $item[0] }
870 value : /\d+/ { bless {__VALUE__=>$item[1]}, $item[0] }
871 Note that each node in the tree is blessed into a class of the same
873 name as the rule itself. This makes it easy to build object-oriented
874 processors for the parse-trees that the grammar produces. Note too that
875 the last two rules produce special objects with the single attribute
876 '__VALUE__'. This is because they consist solely of a single terminal.
877 This autoaction-ed grammar would then produce a parse tree in a data
879 structure like this:
880 {
882 file => {
883 command => {
884 [ get => {
885 identifier => { __VALUE__ => 'a' },
886 },
887 set => {
888 identifier => { __VALUE__ => 'b' },
889 value => { __VALUE__ => '7' },
890 },
891 vet => {
892 identifier => { __VALUE__ => 'b' },
893 value => { __VALUE__ => '7' },
894 },
895 ],
896 },
897 }
898 }
899 (except, of course, that each nested hash would also be blessed into
901 the appropriate class).
902 You can also specify a base class for the "<autotree>" directive. The
904 supplied prefix will be prepended to the rule names when creating tree
905 nodes. The following are equivalent:
906 <autotree:MyBase::Class>
908 <autotree:MyBase::Class::>
909 And will produce a root node blessed into the "MyBase::Class::file"
911 package in the example above.
912 Autostubbing
914 Normally, if a subrule appears in some production, but no rule of that
915 name is ever defined in the grammar, the production which refers to the
916 non-existent subrule fails immediately. This typically occurs as a
917 result of misspellings, and is a sufficiently common occurrence that a
918 warning is generated for such situations.
919 However, when prototyping a grammar it is sometimes useful to be able
921 to use subrules before a proper specification of them is really
922 possible. For example, a grammar might include a section like:
923 function_call: identifier '(' arg(s?) ')'
925 identifier: /[a-z]\w*/i
927 where the possible format of an argument is sufficiently complex that
929 it is not worth specifying in full until the general function call
930 syntax has been debugged. In this situation it is convenient to leave
931 the real rule "arg" undefined and just slip in a placeholder (or
932 "stub"):
933 arg: 'arg'
935 so that the function call syntax can be tested with dummy input such
937 as:
938 f0()
940 f1(arg)
941 f2(arg arg)
942 f3(arg arg arg)
943 et cetera.
945 Early in prototyping, many such "stubs" may be required, so
947 "Parse::RecDescent" provides a means of automating their definition.
948 If the variable $::RD_AUTOSTUB is defined when a parser is built, a
949 subrule reference to any non-existent rule (say, "subrule"), will cause
950 a "stub" rule to be automatically defined in the generated parser. If
951 "$::RD_AUTOSTUB eq '1'" or is false, a stub rule of the form:
952 subrule: 'subrule'
954 will be generated. The special-case for a value of '1' is to allow the
956 use of the perl -s with -RD_AUTOSTUB without generating "subrule: '1'"
957 per below. If $::RD_AUTOSTUB is true, a stub rule of the form:
958 subrule: $::RD_AUTOSTUB
960 will be generated. $::RD_AUTOSTUB must contain a valid production
962 item, no checking is performed. No lazy evaluation of $::RD_AUTOSTUB
963 is performed, it is evaluated at the time the Parser is generated.
964 Hence, with $::RD_AUTOSTUB defined, it is possible to only partially
966 specify a grammar, and then "fake" matches of the unspecified
967 (sub)rules by just typing in their name, or a literal value that was
968 assigned to $::RD_AUTOSTUB.
969 Look-ahead
971 If a subrule, token, or action is prefixed by "...", then it is treated
972 as a "look-ahead" request. That means that the current production can
973 (as usual) only succeed if the specified item is matched, but that the
974 matching does not consume any of the text being parsed. This is very
975 similar to the "/(?=...)/" look-ahead construct in Perl patterns. Thus,
976 the rule:
977 inner_word: word ...word
979 will match whatever the subrule "word" matches, provided that match is
981 followed by some more text which subrule "word" would also match
982 (although this second substring is not actually consumed by
983 "inner_word")
984 Likewise, a "...!" prefix, causes the following item to succeed
986 (without consuming any text) if and only if it would normally fail.
987 Hence, a rule such as:
988 identifier: ...!keyword ...!'_' /[A-Za-z_]\w*/
990 matches a string of characters which satisfies the pattern
992 "/[A-Za-z_]\w*/", but only if the same sequence of characters would not
993 match either subrule "keyword" or the literal token '_'.
994 Sequences of look-ahead prefixes accumulate, multiplying their positive
996 and/or negative senses. Hence:
997 inner_word: word ...!......!word
999 is exactly equivalent to the original example above (a warning is
1001 issued in cases like these, since they often indicate something left
1002 out, or misunderstood).
1003 Note that actions can also be treated as look-aheads. In such cases,
1005 the state of the parser text (in the local variable $text) after the
1006 look-ahead action is guaranteed to be identical to its state before the
1007 action, regardless of how it's changed within the action (unless you
1008 actually undefine $text, in which case you get the disaster you deserve
1009 :-).
1010 Directives
1012 Directives are special pre-defined actions which may be used to alter
1013 the behaviour of the parser. There are currently twenty-three
1014 directives: "<commit>", "<uncommit>", "<reject>", "<score>",
1015 "<autoscore>", "<skip>", "<resync>", "<error>", "<warn>", "<hint>",
1016 "<trace_build>", "<trace_parse>", "<nocheck>", "<rulevar>",
1017 "<matchrule>", "<leftop>", "<rightop>", "<defer>", "<nocheck>",
1018 "<perl_quotelike>", "<perl_codeblock>", "<perl_variable>", and
1019 "<token>".
1020 Committing and uncommitting
1022 The "<commit>" and "<uncommit>" directives permit the recursive
1023 descent of the parse tree to be pruned (or "cut") for efficiency.
1024 Within a rule, a "<commit>" directive instructs the rule to ignore
1025 subsequent productions if the current production fails. For
1026 example:
1027 command: 'find' <commit> filename
1029 | 'open' <commit> filename
1030 | 'move' filename filename
1031 Clearly, if the leading token 'find' is matched in the first
1033 production but that production fails for some other reason, then
1034 the remaining productions cannot possibly match. The presence of
1035 the "<commit>" causes the "command" rule to fail immediately if an
1036 invalid "find" command is found, and likewise if an invalid "open"
1037 command is encountered.
1038 It is also possible to revoke a previous commitment. For example:
1040 if_statement: 'if' <commit> condition
1042 'then' block <uncommit>
1043 'else' block
1044 | 'if' <commit> condition
1045 'then' block
1046 In this case, a failure to find an "else" block in the first
1048 production shouldn't preclude trying the second production, but a
1049 failure to find a "condition" certainly should.
1050 As a special case, any production in which the first item is an
1052 "<uncommit>" immediately revokes a preceding "<commit>" (even
1053 though the production would not otherwise have been tried). For
1054 example, in the rule:
1055 request: 'explain' expression
1057 | 'explain' <commit> keyword
1058 | 'save'
1059 | 'quit'
1060 | <uncommit> term '?'
1061 if the text being matched was "explain?", and the first two
1063 productions failed, then the "<commit>" in production two would
1064 cause productions three and four to be skipped, but the leading
1065 "<uncommit>" in the production five would allow that production to
1066 attempt a match.
1067 Note in the preceding example, that the "<commit>" was only placed
1069 in production two. If production one had been:
1070 request: 'explain' <commit> expression
1072 then production two would be (inappropriately) skipped if a leading
1074 "explain..." was encountered.
1075 Both "<commit>" and "<uncommit>" directives always succeed, and
1077 their value is always 1.
1078 Rejecting a production
1080 The "<reject>" directive immediately causes the current production
1081 to fail (it is exactly equivalent to, but more obvious than, the
1082 action "{undef}"). A "<reject>" is useful when it is desirable to
1083 get the side effects of the actions in one production, without
1084 prejudicing a match by some other production later in the rule. For
1085 example, to insert tracing code into the parse:
1086 complex_rule: { print "In complex rule...\n"; } <reject>
1088 complex_rule: simple_rule '+' 'i' '*' simple_rule
1090 | 'i' '*' simple_rule
1091 | simple_rule
1092 It is also possible to specify a conditional rejection, using the
1094 form "<reject:condition>", which only rejects if the specified
1095 condition is true. This form of rejection is exactly equivalent to
1096 the action "{(condition)?undef:1}>". For example:
1097 command: save_command
1099 | restore_command
1100 | <reject: defined $::tolerant> { exit }
1101 | <error: Unknown command. Ignored.>
1102 A "<reject>" directive never succeeds (and hence has no associated
1104 value). A conditional rejection may succeed (if its condition is
1105 not satisfied), in which case its value is 1.
1106 As an extra optimization, "Parse::RecDescent" ignores any
1108 production which begins with an unconditional "<reject>" directive,
1109 since any such production can never successfully match or have any
1110 useful side-effects. A level 1 warning is issued in all such cases.
1111 Note that productions beginning with conditional "<reject:...>"
1113 directives are never "optimized away" in this manner, even if they
1114 are always guaranteed to fail (for example: "<reject:1>")
1115 Due to the way grammars are parsed, there is a minor restriction on
1117 the condition of a conditional "<reject:...>": it cannot contain
1118 any raw '<' or '>' characters. For example:
1119 line: cmd <reject: $thiscolumn > max> data
1121 results in an error when a parser is built from this grammar (since
1123 the grammar parser has no way of knowing whether the first > is a
1124 "less than" or the end of the "<reject:...>".
1125 To overcome this problem, put the condition inside a do{} block:
1127 line: cmd <reject: do{$thiscolumn > max}> data
1129 Note that the same problem may occur in other directives that take
1131 arguments. The same solution will work in all cases.
1132 Skipping between terminals
1134 The "<skip>" directive enables the terminal prefix used in a
1135 production to be changed. For example:
1136 OneLiner: Command <skip:'[ \t]*'> Arg(s) /;/
1138 causes only blanks and tabs to be skipped before terminals in the
1140 "Arg" subrule (and any of its subrules>, and also before the final
1141 "/;/" terminal. Once the production is complete, the previous
1142 terminal prefix is reinstated. Note that this implies that distinct
1143 productions of a rule must reset their terminal prefixes
1144 individually.
1145 The "<skip>" directive evaluates to the previous terminal prefix,
1147 so it's easy to reinstate a prefix later in a production:
1148 Command: <skip:","> CSV(s) <skip:$item[1]> Modifier
1150 The value specified after the colon is interpolated into a pattern,
1152 so all of the following are equivalent (though their efficiency
1153 increases down the list):
1154 <skip: "$colon|$comma"> # ASSUMING THE VARS HOLD THE OBVIOUS VALUES
1156 <skip: ':|,'>
1158 <skip: q{[:,]}>
1160 <skip: qr/[:,]/>
1162 There is no way of directly setting the prefix for an entire rule,
1164 except as follows:
1165 Rule: <skip: '[ \t]*'> Prod1
1167 | <skip: '[ \t]*'> Prod2a Prod2b
1168 | <skip: '[ \t]*'> Prod3
1169 or, better:
1171 Rule: <skip: '[ \t]*'>
1173 (
1174 Prod1
1175 | Prod2a Prod2b
1176 | Prod3
1177 )
1178 The skip pattern is passed down to subrules, so setting the skip
1180 for the top-level rule as described above actually sets the prefix
1181 for the entire grammar (provided that you only call the method
1182 corresponding to the top-level rule itself). Alternatively, or if
1183 you have more than one top-level rule in your grammar, you can
1184 provide a global "<skip>" directive prior to defining any rules in
1185 the grammar. These are the preferred alternatives to setting
1186 $Parse::RecDescent::skip.
1187 Additionally, using "<skip>" actually allows you to have a
1189 completely dynamic skipping behaviour. For example:
1190 Rule_with_dynamic_skip: <skip: $::skip_pattern> Rule
1192 Then you can set $::skip_pattern before invoking
1194 "Rule_with_dynamic_skip" and have it skip whatever you specified.
1195 Note: Up to release 1.51 of Parse::RecDescent, an entirely
1197 different mechanism was used for specifying terminal prefixes. The
1198 current method is not backwards-compatible with that early
1199 approach. The current approach is stable and will not change again.
1200 Note: the global "<skip>" directive added in 1.967_004 did not
1202 interpolate the pattern argument, instead the pattern was placed
1203 inside of single quotes and then interpolated. This behavior was
1204 changed in 1.967_010 so that all "<skip>" directives behavior
1205 similarly.
1206 Resynchronization
1208 The "<resync>" directive provides a visually distinctive means of
1209 consuming some of the text being parsed, usually to skip an
1210 erroneous input. In its simplest form "<resync>" simply consumes
1211 text up to and including the next newline ("\n") character,
1212 succeeding only if the newline is found, in which case it causes
1213 its surrounding rule to return zero on success.
1214 In other words, a "<resync>" is exactly equivalent to the token
1216 "/[^\n]*\n/" followed by the action "{ $return = 0 }" (except that
1217 productions beginning with a "<resync>" are ignored when generating
1218 error messages). A typical use might be:
1219 script : command(s)
1221 command: save_command
1223 | restore_command
1224 | <resync> # TRY NEXT LINE, IF POSSIBLE
1225 It is also possible to explicitly specify a resynchronization
1227 pattern, using the "<resync:pattern>" variant. This version
1228 succeeds only if the specified pattern matches (and consumes) the
1229 parsed text. In other words, "<resync:pattern>" is exactly
1230 equivalent to the token "/pattern/" (followed by a
1231 "{ $return = 0 }" action). For example, if commands were terminated
1232 by newlines or semi-colons:
1233 command: save_command
1235 | restore_command
1236 | <resync:[^;\n]*[;\n]>
1237 The value of a successfully matched "<resync>" directive (of either
1239 type) is the text that it consumed. Note, however, that since the
1240 directive also sets $return, a production consisting of a lone
1241 "<resync>" succeeds but returns the value zero (which a calling
1242 rule may find useful to distinguish between "true" matches and
1243 "tolerant" matches). Remember that returning a zero value
1244 indicates that the rule succeeded (since only an "undef" denotes
1245 failure within "Parse::RecDescent" parsers.
1246 Error handling
1248 The "<error>" directive provides automatic or user-defined
1249 generation of error messages during a parse. In its simplest form
1250 "<error>" prepares an error message based on the mismatch between
1251 the last item expected and the text which cause it to fail. For
1252 example, given the rule:
1253 McCoy: curse ',' name ', I'm a doctor, not a' a_profession '!'
1255 | pronoun 'dead,' name '!'
1256 | <error>
1257 the following strings would produce the following messages:
1259 "Amen, Jim!"
1261 ERROR (line 1): Invalid McCoy: Expected curse or pronoun
1262 not found
1263 "Dammit, Jim, I'm a doctor!"
1265 ERROR (line 1): Invalid McCoy: Expected ", I'm a doctor, not a"
1266 but found ", I'm a doctor!" instead
1267 "He's dead,\n"
1269 ERROR (line 2): Invalid McCoy: Expected name not found
1270 "He's alive!"
1272 ERROR (line 1): Invalid McCoy: Expected 'dead,' but found
1273 "alive!" instead
1274 "Dammit, Jim, I'm a doctor, not a pointy-eared Vulcan!"
1276 ERROR (line 1): Invalid McCoy: Expected a profession but found
1277 "pointy-eared Vulcan!" instead
1278 Note that, when autogenerating error messages, all underscores in
1280 any rule name used in a message are replaced by single spaces (for
1281 example "a_production" becomes "a production"). Judicious choice of
1282 rule names can therefore considerably improve the readability of
1283 automatic error messages (as well as the maintainability of the
1284 original grammar).
1285 If the automatically generated error is not sufficient, it is
1287 possible to provide an explicit message as part of the error
1288 directive. For example:
1289 Spock: "Fascinating ',' (name | 'Captain') '.'
1291 | "Highly illogical, doctor."
1292 | <error: He never said that!>
1293 which would result in all failures to parse a "Spock" subrule
1295 printing the following message:
1296 ERROR (line <N>): Invalid Spock: He never said that!
1298 The error message is treated as a "qq{...}" string and interpolated
1300 when the error is generated (not when the directive is specified!).
1301 Hence:
1302 <error: Mystical error near "$text">
1304 would correctly insert the ambient text string which caused the
1306 error.
1307 There are two other forms of error directive: "<error?>" and
1309 "<error?: msg>". These behave just like "<error>" and
1310 "<error: msg>" respectively, except that they are only triggered if
1311 the rule is "committed" at the time they are encountered. For
1312 example:
1313 Scotty: "Ya kenna change the Laws of Phusics," <commit> name
1315 | name <commit> ',' 'she's goanta blaw!'
1316 | <error?>
1317 will only generate an error for a string beginning with "Ya kenna
1319 change the Laws o' Phusics," or a valid name, but which still fails
1320 to match the corresponding production. That is,
1321 "$parser->Scotty("Aye, Cap'ain")" will fail silently (since neither
1322 production will "commit" the rule on that input), whereas
1323 "$parser->Scotty("Mr Spock, ah jest kenna do'ut!")" will fail with
1324 the error message:
1325 ERROR (line 1): Invalid Scotty: expected 'she's goanta blaw!'
1327 but found 'I jest kenna do'ut!' instead.
1328 since in that case the second production would commit after
1330 matching the leading name.
1331 Note that to allow this behaviour, all "<error>" directives which
1333 are the first item in a production automatically uncommit the rule
1334 just long enough to allow their production to be attempted (that
1335 is, when their production fails, the commitment is reinstated so
1336 that subsequent productions are skipped).
1337 In order to permanently uncommit the rule before an error message,
1339 it is necessary to put an explicit "<uncommit>" before the
1340 "<error>". For example:
1341 line: 'Kirk:' <commit> Kirk
1343 | 'Spock:' <commit> Spock
1344 | 'McCoy:' <commit> McCoy
1345 | <uncommit> <error?> <reject>
1346 | <resync>
1347 Error messages generated by the various "<error...>" directives are
1349 not displayed immediately. Instead, they are "queued" in a buffer
1350 and are only displayed once parsing ultimately fails. Moreover,
1351 "<error...>" directives that cause one production of a rule to fail
1352 are automatically removed from the message queue if another
1353 production subsequently causes the entire rule to succeed. This
1354 means that you can put "<error...>" directives wherever useful
1355 diagnosis can be done, and only those associated with actual parser
1356 failure will ever be displayed. Also see "GOTCHAS".
1357 As a general rule, the most useful diagnostics are usually
1359 generated either at the very lowest level within the grammar, or at
1360 the very highest. A good rule of thumb is to identify those
1361 subrules which consist mainly (or entirely) of terminals, and then
1362 put an "<error...>" directive at the end of any other rule which
1363 calls one or more of those subrules.
1364 There is one other situation in which the output of the various
1366 types of error directive is suppressed; namely, when the rule
1367 containing them is being parsed as part of a "look-ahead" (see
1368 "Look-ahead"). In this case, the error directive will still cause
1369 the rule to fail, but will do so silently.
1370 An unconditional "<error>" directive always fails (and hence has no
1372 associated value). This means that encountering such a directive
1373 always causes the production containing it to fail. Hence an
1374 "<error>" directive will inevitably be the last (useful) item of a
1375 rule (a level 3 warning is issued if a production contains items
1376 after an unconditional "<error>" directive).
1377 An "<error?>" directive will succeed (that is: fail to fail :-), if
1379 the current rule is uncommitted when the directive is encountered.
1380 In that case the directive's associated value is zero. Hence, this
1381 type of error directive can be used before the end of a production.
1382 For example:
1383 command: 'do' <commit> something
1385 | 'report' <commit> something
1386 | <error?: Syntax error> <error: Unknown command>
1387 Warning: The "<error?>" directive does not mean "always fail (but
1389 do so silently unless committed)". It actually means "only fail
1390 (and report) if committed, otherwise succeed". To achieve the "fail
1391 silently if uncommitted" semantics, it is necessary to use:
1392 rule: item <commit> item(s)
1394 | <error?> <reject> # FAIL SILENTLY UNLESS COMMITTED
1395 However, because people seem to expect a lone "<error?>" directive
1397 to work like this:
1398 rule: item <commit> item(s)
1400 | <error?: Error message if committed>
1401 | <error: Error message if uncommitted>
1402 Parse::RecDescent automatically appends a "<reject>" directive if
1404 the "<error?>" directive is the only item in a production. A level
1405 2 warning (see below) is issued when this happens.
1406 The level of error reporting during both parser construction and
1408 parsing is controlled by the presence or absence of four global
1409 variables: $::RD_ERRORS, $::RD_WARN, $::RD_HINT, and <$::RD_TRACE>.
1410 If $::RD_ERRORS is defined (and, by default, it is) then fatal
1411 errors are reported.
1412 Whenever $::RD_WARN is defined, certain non-fatal problems are also
1414 reported.
1415 Warnings have an associated "level": 1, 2, or 3. The higher the
1417 level, the more serious the warning. The value of the corresponding
1418 global variable ($::RD_WARN) determines the lowest level of warning
1419 to be displayed. Hence, to see all warnings, set $::RD_WARN to 1.
1420 To see only the most serious warnings set $::RD_WARN to 3. By
1421 default $::RD_WARN is initialized to 3, ensuring that serious but
1422 non-fatal errors are automatically reported.
1423 There is also a grammar directive to turn on warnings from within
1425 the grammar: "<warn>". It takes an optional argument, which
1426 specifies the warning level: "<warn: 2>".
1427 See "DIAGNOSTICS" for a list of the various error and warning
1429 messages that Parse::RecDescent generates when these two variables
1430 are defined.
1431 Defining any of the remaining variables (which are not defined by
1433 default) further increases the amount of information reported.
1434 Defining $::RD_HINT causes the parser generator to offer more
1435 detailed analyses and hints on both errors and warnings. Note that
1436 setting $::RD_HINT at any point automagically sets $::RD_WARN to 1.
1437 There is also a "<hint>" directive, which can be hard-coded into a
1438 grammar.
1439 Defining $::RD_TRACE causes the parser generator and the parser to
1441 report their progress to STDERR in excruciating detail (although,
1442 without hints unless $::RD_HINT is separately defined). This detail
1443 can be moderated in only one respect: if $::RD_TRACE has an integer
1444 value (N) greater than 1, only the N characters of the "current
1445 parsing context" (that is, where in the input string we are at any
1446 point in the parse) is reported at any time.
1447 $::RD_TRACE is mainly useful for debugging a grammar that isn't
1449 behaving as you expected it to. To this end, if $::RD_TRACE is
1450 defined when a parser is built, any actual parser code which is
1451 generated is also written to a file named "RD_TRACE" in the local
1452 directory.
1453 There are two directives associated with the $::RD_TRACE variable.
1455 If a grammar contains a "<trace_build>" directive anywhere in its
1456 specification, $::RD_TRACE is turned on during the parser
1457 construction phase. If a grammar contains a "<trace_parse>"
1458 directive anywhere in its specification, $::RD_TRACE is turned on
1459 during any parse the parser performs.
1460 Note that the four variables belong to the "main" package, which
1462 makes them easier to refer to in the code controlling the parser,
1463 and also makes it easy to turn them into command line flags
1464 ("-RD_ERRORS", "-RD_WARN", "-RD_HINT", "-RD_TRACE") under perl -s.
1465 The corresponding directives are useful to "hardwire" the various
1467 debugging features into a particular grammar (rather than having to
1468 set and reset external variables).
1469 Redirecting diagnostics
1471 The diagnostics provided by the tracing mechanism always go to
1472 STDERR. If you need them to go elsewhere, localize and reopen
1473 STDERR prior to the parse.
1474 For example:
1476 {
1478 local *STDERR = IO::File->new(">$filename") or die $!;
1479 my $result = $parser->startrule($text);
1481 }
1482 Consistency checks
1484 Whenever a parser is build, Parse::RecDescent carries out a number
1485 of (potentially expensive) consistency checks. These include:
1486 verifying that the grammar is not left-recursive and that no rules
1487 have been left undefined.
1488 These checks are important safeguards during development, but
1490 unnecessary overheads when the grammar is stable and ready to be
1491 deployed. So Parse::RecDescent provides a directive to disable
1492 them: "<nocheck>".
1493 If a grammar contains a "<nocheck>" directive anywhere in its
1495 specification, the extra compile-time checks are by-passed.
1496 Specifying local variables
1498 It is occasionally convenient to specify variables which are local
1499 to a single rule. This may be achieved by including a
1500 "<rulevar:...>" directive anywhere in the rule. For example:
1501 markup: <rulevar: $tag>
1503 markup: tag {($tag=$item[1]) =~ s/^<|>$//g} body[$tag]
1505 The example "<rulevar: $tag>" directive causes a "my" variable
1507 named $tag to be declared at the start of the subroutine
1508 implementing the "markup" rule (that is, before the first
1509 production, regardless of where in the rule it is specified).
1510 Specifically, any directive of the form: "<rulevar:text>" causes a
1512 line of the form "my text;" to be added at the beginning of the
1513 rule subroutine, immediately after the definitions of the following
1514 local variables:
1515 $thisparser $commit
1517 $thisrule @item
1518 $thisline @arg
1519 $text %arg
1520 This means that the following "<rulevar>" directives work as
1522 expected:
1523 <rulevar: $count = 0 >
1525 <rulevar: $firstarg = $arg[0] || '' >
1527 <rulevar: $myItems = \@item >
1529 <rulevar: @context = ( $thisline, $text, @arg ) >
1531 <rulevar: ($name,$age) = $arg{"name","age"} >
1533 If a variable that is also visible to subrules is required, it
1535 needs to be "local"'d, not "my"'d. "rulevar" defaults to "my", but
1536 if "local" is explicitly specified:
1537 <rulevar: local $count = 0 >
1539 then a "local"-ized variable is declared instead, and will be
1541 available within subrules.
1542 Note however that, because all such variables are "my" variables,
1544 their values do not persist between match attempts on a given rule.
1545 To preserve values between match attempts, values can be stored
1546 within the "local" member of the $thisrule object:
1547 countedrule: { $thisrule->{"local"}{"count"}++ }
1549 <reject>
1550 | subrule1
1551 | subrule2
1552 | <reject: $thisrule->{"local"}{"count"} == 1>
1553 subrule3
1554 When matching a rule, each "<rulevar>" directive is matched as if
1556 it were an unconditional "<reject>" directive (that is, it causes
1557 any production in which it appears to immediately fail to match).
1558 For this reason (and to improve readability) it is usual to specify
1559 any "<rulevar>" directive in a separate production at the start of
1560 the rule (this has the added advantage that it enables
1561 "Parse::RecDescent" to optimize away such productions, just as it
1562 does for the "<reject>" directive).
1563 Dynamically matched rules
1565 Because regexes and double-quoted strings are interpolated, it is
1566 relatively easy to specify productions with "context sensitive"
1567 tokens. For example:
1568 command: keyword body "end $item[1]"
1570 which ensures that a command block is bounded by a "<keyword>...end
1572 <same keyword>" pair.
1573 Building productions in which subrules are context sensitive is
1575 also possible, via the "<matchrule:...>" directive. This directive
1576 behaves identically to a subrule item, except that the rule which
1577 is invoked to match it is determined by the string specified after
1578 the colon. For example, we could rewrite the "command" rule like
1579 this:
1580 command: keyword <matchrule:body> "end $item[1]"
1582 Whatever appears after the colon in the directive is treated as an
1584 interpolated string (that is, as if it appeared in "qq{...}"
1585 operator) and the value of that interpolated string is the name of
1586 the subrule to be matched.
1587 Of course, just putting a constant string like "body" in a
1589 "<matchrule:...>" directive is of little interest or benefit. The
1590 power of directive is seen when we use a string that interpolates
1591 to something interesting. For example:
1592 command: keyword <matchrule:$item[1]_body> "end $item[1]"
1594 keyword: 'while' | 'if' | 'function'
1596 while_body: condition block
1598 if_body: condition block ('else' block)(?)
1600 function_body: arglist block
1602 Now the "command" rule selects how to proceed on the basis of the
1604 keyword that is found. It is as if "command" were declared:
1605 command: 'while' while_body "end while"
1607 | 'if' if_body "end if"
1608 | 'function' function_body "end function"
1609 When a "<matchrule:...>" directive is used as a repeated subrule,
1611 the rule name expression is "late-bound". That is, the name of the
1612 rule to be called is re-evaluated each time a match attempt is
1613 made. Hence, the following grammar:
1614 { $::species = 'dogs' }
1616 pair: 'two' <matchrule:$::species>(s)
1618 dogs: /dogs/ { $::species = 'cats' }
1620 cats: /cats/
1622 will match the string "two dogs cats cats" completely, whereas it
1624 will only match the string "two dogs dogs dogs" up to the eighth
1625 letter. If the rule name were "early bound" (that is, evaluated
1626 only the first time the directive is encountered in a production),
1627 the reverse behaviour would be expected.
1628 Note that the "matchrule" directive takes a string that is to be
1630 treated as a rule name, not as a rule invocation. That is, it's
1631 like a Perl symbolic reference, not an "eval". Just as you can say:
1632 $subname = 'foo';
1634 # and later...
1636 &{$foo}(@args);
1638 but not:
1640 $subname = 'foo(@args)';
1642 # and later...
1644 &{$foo};
1646 likewise you can say:
1648 $rulename = 'foo';
1650 # and in the grammar...
1652 <matchrule:$rulename>[@args]
1654 but not:
1656 $rulename = 'foo[@args]';
1658 # and in the grammar...
1660 <matchrule:$rulename>
1662 Deferred actions
1664 The "<defer:...>" directive is used to specify an action to be
1665 performed when (and only if!) the current production ultimately
1666 succeeds.
1667 Whenever a "<defer:...>" directive appears, the code it specifies
1669 is converted to a closure (an anonymous subroutine reference) which
1670 is queued within the active parser object. Note that, because the
1671 deferred code is converted to a closure, the values of any "local"
1672 variable (such as $text, <@item>, etc.) are preserved until the
1673 deferred code is actually executed.
1674 If the parse ultimately succeeds and the production in which the
1676 "<defer:...>" directive was evaluated formed part of the successful
1677 parse, then the deferred code is executed immediately before the
1678 parse returns. If however the production which queued a deferred
1679 action fails, or one of the higher-level rules which called that
1680 production fails, then the deferred action is removed from the
1681 queue, and hence is never executed.
1682 For example, given the grammar:
1684 sentence: noun trans noun
1686 | noun intrans
1687 noun: 'the dog'
1689 { print "$item[1]\t(noun)\n" }
1690 | 'the meat'
1691 { print "$item[1]\t(noun)\n" }
1692 trans: 'ate'
1694 { print "$item[1]\t(transitive)\n" }
1695 intrans: 'ate'
1697 { print "$item[1]\t(intransitive)\n" }
1698 | 'barked'
1699 { print "$item[1]\t(intransitive)\n" }
1700 then parsing the sentence "the dog ate" would produce the output:
1702 the dog (noun)
1704 ate (transitive)
1705 the dog (noun)
1706 ate (intransitive)
1707 This is because, even though the first production of "sentence"
1709 ultimately fails, its initial subrules "noun" and "trans" do match,
1710 and hence they execute their associated actions. Then the second
1711 production of "sentence" succeeds, causing the actions of the
1712 subrules "noun" and "intrans" to be executed as well.
1713 On the other hand, if the actions were replaced by "<defer:...>"
1715 directives:
1716 sentence: noun trans noun
1718 | noun intrans
1719 noun: 'the dog'
1721 <defer: print "$item[1]\t(noun)\n" >
1722 | 'the meat'
1723 <defer: print "$item[1]\t(noun)\n" >
1724 trans: 'ate'
1726 <defer: print "$item[1]\t(transitive)\n" >
1727 intrans: 'ate'
1729 <defer: print "$item[1]\t(intransitive)\n" >
1730 | 'barked'
1731 <defer: print "$item[1]\t(intransitive)\n" >
1732 the output would be:
1734 the dog (noun)
1736 ate (intransitive)
1737 since deferred actions are only executed if they were evaluated in
1739 a production which ultimately contributes to the successful parse.
1740 In this case, even though the first production of "sentence" caused
1742 the subrules "noun" and "trans" to match, that production
1743 ultimately failed and so the deferred actions queued by those
1744 subrules were subsequently discarded. The second production then
1745 succeeded, causing the entire parse to succeed, and so the deferred
1746 actions queued by the (second) match of the "noun" subrule and the
1747 subsequent match of "intrans" are preserved and eventually
1748 executed.
1749 Deferred actions provide a means of improving the performance of a
1751 parser, by only executing those actions which are part of the final
1752 parse-tree for the input data.
1753 Alternatively, deferred actions can be viewed as a mechanism for
1755 building (and executing) a customized subroutine corresponding to
1756 the given input data, much in the same way that autoactions (see
1757 "Autoactions") can be used to build a customized data structure for
1758 specific input.
1759 Whether or not the action it specifies is ever executed, a
1761 "<defer:...>" directive always succeeds, returning the number of
1762 deferred actions currently queued at that point.
1763 Parsing Perl
1765 Parse::RecDescent provides limited support for parsing subsets of
1766 Perl, namely: quote-like operators, Perl variables, and complete
1767 code blocks.
1768 The "<perl_quotelike>" directive can be used to parse any Perl
1770 quote-like operator: 'a string', "m/a pattern/", "tr{ans}{lation}",
1771 etc. It does this by calling Text::Balanced::quotelike().
1772 If a quote-like operator is found, a reference to an array of eight
1774 elements is returned. Those elements are identical to the last
1775 eight elements returned by Text::Balanced::extract_quotelike() in
1776 an array context, namely:
1777 [0] the name of the quotelike operator -- 'q', 'qq', 'm', 's', 'tr'
1779 -- if the operator was named; otherwise "undef",
1780 [1] the left delimiter of the first block of the operation,
1782 [2] the text of the first block of the operation (that is, the
1784 contents of a quote, the regex of a match, or substitution or
1785 the target list of a translation),
1786 [3] the right delimiter of the first block of the operation,
1788 [4] the left delimiter of the second block of the operation if
1790 there is one (that is, if it is a "s", "tr", or "y"); otherwise
1791 "undef",
1792 [5] the text of the second block of the operation if there is one
1794 (that is, the replacement of a substitution or the translation
1795 list of a translation); otherwise "undef",
1796 [6] the right delimiter of the second block of the operation (if
1798 any); otherwise "undef",
1799 [7] the trailing modifiers on the operation (if any); otherwise
1801 "undef".
1802 If a quote-like expression is not found, the directive fails with
1804 the usual "undef" value.
1805 The "<perl_variable>" directive can be used to parse any Perl
1807 variable: $scalar, @array, %hash, $ref->{field}[$index], etc. It
1808 does this by calling Text::Balanced::extract_variable().
1809 If the directive matches text representing a valid Perl variable
1811 specification, it returns that text. Otherwise it fails with the
1812 usual "undef" value.
1813 The "<perl_codeblock>" directive can be used to parse curly-brace-
1815 delimited block of Perl code, such as: { $a = 1; f() =~ m/pat/; }.
1816 It does this by calling Text::Balanced::extract_codeblock().
1817 If the directive matches text representing a valid Perl code block,
1819 it returns that text. Otherwise it fails with the usual "undef"
1820 value.
1821 You can also tell it what kind of brackets to use as the outermost
1823 delimiters. For example:
1824 arglist: <perl_codeblock ()>
1826 causes an arglist to match a perl code block whose outermost
1828 delimiters are "(...)" (rather than the default "{...}").
1829 Constructing tokens
1831 Eventually, Parse::RecDescent will be able to parse tokenized
1832 input, as well as ordinary strings. In preparation for this joyous
1833 day, the "<token:...>" directive has been provided. This directive
1834 creates a token which will be suitable for input to a
1835 Parse::RecDescent parser (when it eventually supports tokenized
1836 input).
1837 The text of the token is the value of the immediately preceding
1839 item in the production. A "<token:...>" directive always succeeds
1840 with a return value which is the hash reference that is the new
1841 token. It also sets the return value for the production to that
1842 hash ref.
1843 The "<token:...>" directive makes it easy to build a
1845 Parse::RecDescent-compatible lexer in Parse::RecDescent:
1846 my $lexer = new Parse::RecDescent q
1848 {
1849 lex: token(s)
1850 token: /a\b/ <token:INDEF>
1852 | /the\b/ <token:DEF>
1853 | /fly\b/ <token:NOUN,VERB>
1854 | /[a-z]+/i { lc $item[1] } <token:ALPHA>
1855 | <error: Unknown token>
1856 };
1858 which will eventually be able to be used with a regular
1860 Parse::RecDescent grammar:
1861 my $parser = new Parse::RecDescent q
1863 {
1864 startrule: subrule1 subrule 2
1865 # ETC...
1867 };
1868 either with a pre-lexing phase:
1870 $parser->startrule( $lexer->lex($data) );
1872 or with a lex-on-demand approach:
1874 $parser->startrule( sub{$lexer->token(\$data)} );
1876 But at present, only the "<token:...>" directive is actually
1878 implemented. The rest is vapourware.
1879 Specifying operations
1881 One of the commonest requirements when building a parser is to
1882 specify binary operators. Unfortunately, in a normal grammar, the
1883 rules for such things are awkward:
1884 disjunction: conjunction ('or' conjunction)(s?)
1886 { $return = [ $item[1], @{$item[2]} ] }
1887 conjunction: atom ('and' atom)(s?)
1889 { $return = [ $item[1], @{$item[2]} ] }
1890 or inefficient:
1892 disjunction: conjunction 'or' disjunction
1894 { $return = [ $item[1], @{$item[2]} ] }
1895 | conjunction
1896 { $return = [ $item[1] ] }
1897 conjunction: atom 'and' conjunction
1899 { $return = [ $item[1], @{$item[2]} ] }
1900 | atom
1901 { $return = [ $item[1] ] }
1902 and either way is ugly and hard to get right.
1904 The "<leftop:...>" and "<rightop:...>" directives provide an easier
1906 way of specifying such operations. Using "<leftop:...>" the above
1907 examples become:
1908 disjunction: <leftop: conjunction 'or' conjunction>
1910 conjunction: <leftop: atom 'and' atom>
1911 The "<leftop:...>" directive specifies a left-associative binary
1913 operator. It is specified around three other grammar elements
1914 (typically subrules or terminals), which match the left operand,
1915 the operator itself, and the right operand respectively.
1916 A "<leftop:...>" directive such as:
1918 disjunction: <leftop: conjunction 'or' conjunction>
1920 is converted to the following:
1922 disjunction: ( conjunction ('or' conjunction)(s?)
1924 { $return = [ $item[1], @{$item[2]} ] } )
1925 In other words, a "<leftop:...>" directive matches the left operand
1927 followed by zero or more repetitions of both the operator and the
1928 right operand. It then flattens the matched items into an anonymous
1929 array which becomes the (single) value of the entire "<leftop:...>"
1930 directive.
1931 For example, an "<leftop:...>" directive such as:
1933 output: <leftop: ident '<<' expr >
1935 when given a string such as:
1937 cout << var << "str" << 3
1939 would match, and $item[1] would be set to:
1941 [ 'cout', 'var', '"str"', '3' ]
1943 In other words:
1945 output: <leftop: ident '<<' expr >
1947 is equivalent to a left-associative operator:
1949 output: ident { $return = [$item[1]] }
1951 | ident '<<' expr { $return = [@item[1,3]] }
1952 | ident '<<' expr '<<' expr { $return = [@item[1,3,5]] }
1953 | ident '<<' expr '<<' expr '<<' expr { $return = [@item[1,3,5,7]] }
1954 # ...etc...
1955 Similarly, the "<rightop:...>" directive takes a left operand, an
1957 operator, and a right operand:
1958 assign: <rightop: var '=' expr >
1960 and converts them to:
1962 assign: ( (var '=' {$return=$item[1]})(s?) expr
1964 { $return = [ @{$item[1]}, $item[2] ] } )
1965 which is equivalent to a right-associative operator:
1967 assign: expr { $return = [$item[1]] }
1969 | var '=' expr { $return = [@item[1,3]] }
1970 | var '=' var '=' expr { $return = [@item[1,3,5]] }
1971 | var '=' var '=' var '=' expr { $return = [@item[1,3,5,7]] }
1972 # ...etc...
1973 Note that for both the "<leftop:...>" and "<rightop:...>"
1975 directives, the directive does not normally return the operator
1976 itself, just a list of the operands involved. This is particularly
1977 handy for specifying lists:
1978 list: '(' <leftop: list_item ',' list_item> ')'
1980 { $return = $item[2] }
1981 There is, however, a problem: sometimes the operator is itself
1983 significant. For example, in a Perl list a comma and a "=>" are
1984 both valid separators, but the "=>" has additional stringification
1985 semantics. Hence it's important to know which was used in each
1986 case.
1987 To solve this problem the "<leftop:...>" and "<rightop:...>"
1989 directives do return the operator(s) as well, under two
1990 circumstances. The first case is where the operator is specified
1991 as a subrule. In that instance, whatever the operator matches is
1992 returned (on the assumption that if the operator is important
1993 enough to have its own subrule, then it's important enough to
1994 return).
1995 The second case is where the operator is specified as a regular
1997 expression. In that case, if the first bracketed subpattern of the
1998 regular expression matches, that matching value is returned (this
1999 is analogous to the behaviour of the Perl "split" function, except
2000 that only the first subpattern is returned).
2001 In other words, given the input:
2003 ( a=>1, b=>2 )
2005 the specifications:
2007 list: '(' <leftop: list_item separator list_item> ')'
2009 separator: ',' | '=>'
2011 or:
2013 list: '(' <leftop: list_item /(,|=>)/ list_item> ')'
2015 cause the list separators to be interleaved with the operands in
2017 the anonymous array in $item[2]:
2018 [ 'a', '=>', '1', ',', 'b', '=>', '2' ]
2020 But the following version:
2022 list: '(' <leftop: list_item /,|=>/ list_item> ')'
2024 returns only the operators:
2026 [ 'a', '1', 'b', '2' ]
2028 Of course, none of the above specifications handle the case of an
2030 empty list, since the "<leftop:...>" and "<rightop:...>" directives
2031 require at least a single right or left operand to match. To
2032 specify that the operator can match "trivially", it's necessary to
2033 add a "(s?)" qualifier to the directive:
2034 list: '(' <leftop: list_item /(,|=>)/ list_item>(s?) ')'
2036 Note that in almost all the above examples, the first and third
2038 arguments of the "<leftop:...>" directive were the same subrule.
2039 That is because "<leftop:...>"'s are frequently used to specify
2040 "separated" lists of the same type of item. To make such lists
2041 easier to specify, the following syntax:
2042 list: element(s /,/)
2044 is exactly equivalent to:
2046 list: <leftop: element /,/ element>
2048 Note that the separator must be specified as a raw pattern (i.e.
2050 not a string or subrule).
2051 Scored productions
2053 By default, Parse::RecDescent grammar rules always accept the first
2054 production that matches the input. But if two or more productions
2055 may potentially match the same input, choosing the first that does
2056 so may not be optimal.
2057 For example, if you were parsing the sentence "time flies like an
2059 arrow", you might use a rule like this:
2060 sentence: verb noun preposition article noun { [@item] }
2062 | adjective noun verb article noun { [@item] }
2063 | noun verb preposition article noun { [@item] }
2064 Each of these productions matches the sentence, but the third one
2066 is the most likely interpretation. However, if the sentence had
2067 been "fruit flies like a banana", then the second production is
2068 probably the right match.
2069 To cater for such situations, the "<score:...>" can be used. The
2071 directive is equivalent to an unconditional "<reject>", except that
2072 it allows you to specify a "score" for the current production. If
2073 that score is numerically greater than the best score of any
2074 preceding production, the current production is cached for later
2075 consideration. If no later production matches, then the cached
2076 production is treated as having matched, and the value of the item
2077 immediately before its "<score:...>" directive is returned as the
2078 result.
2079 In other words, by putting a "<score:...>" directive at the end of
2081 each production, you can select which production matches using
2082 criteria other than specification order. For example:
2083 sentence: verb noun preposition article noun { [@item] } <score: sensible(@item)>
2085 | adjective noun verb article noun { [@item] } <score: sensible(@item)>
2086 | noun verb preposition article noun { [@item] } <score: sensible(@item)>
2087 Now, when each production reaches its respective "<score:...>"
2089 directive, the subroutine "sensible" will be called to evaluate the
2090 matched items (somehow). Once all productions have been tried, the
2091 one which "sensible" scored most highly will be the one that is
2092 accepted as a match for the rule.
2093 The variable $score always holds the current best score of any
2095 production, and the variable $score_return holds the corresponding
2096 return value.
2097 As another example, the following grammar matches lines that may be
2099 separated by commas, colons, or semi-colons. This can be tricky if
2100 a colon-separated line also contains commas, or vice versa. The
2101 grammar resolves the ambiguity by selecting the rule that results
2102 in the fewest fields:
2103 line: seplist[sep=>','] <score: -@{$item[1]}>
2105 | seplist[sep=>':'] <score: -@{$item[1]}>
2106 | seplist[sep=>" "] <score: -@{$item[1]}>
2107 seplist: <skip:""> <leftop: /[^$arg{sep}]*/ "$arg{sep}" /[^$arg{sep}]*/>
2109 Note the use of negation within the "<score:...>" directive to
2111 ensure that the seplist with the most items gets the lowest score.
2112 As the above examples indicate, it is often the case that all
2114 productions in a rule use exactly the same "<score:...>" directive.
2115 It is tedious to have to repeat this identical directive in every
2116 production, so Parse::RecDescent also provides the
2117 "<autoscore:...>" directive.
2118 If an "<autoscore:...>" directive appears in any production of a
2120 rule, the code it specifies is used as the scoring code for every
2121 production of that rule, except productions that already end with
2122 an explicit "<score:...>" directive. Thus the rules above could be
2123 rewritten:
2124 line: <autoscore: -@{$item[1]}>
2126 line: seplist[sep=>',']
2127 | seplist[sep=>':']
2128 | seplist[sep=>" "]
2129 sentence: <autoscore: sensible(@item)>
2132 | verb noun preposition article noun { [@item] }
2133 | adjective noun verb article noun { [@item] }
2134 | noun verb preposition article noun { [@item] }
2135 Note that the "<autoscore:...>" directive itself acts as an
2137 unconditional "<reject>", and (like the "<rulevar:...>" directive)
2138 is pruned at compile-time wherever possible.
2139 Dispensing with grammar checks
2141 During the compilation phase of parser construction,
2142 Parse::RecDescent performs a small number of checks on the grammar
2143 it's given. Specifically it checks that the grammar is not left-
2144 recursive, that there are no "insatiable" constructs of the form:
2145 rule: subrule(s) subrule
2147 and that there are no rules missing (i.e. referred to, but never
2149 defined).
2150 These checks are important during development, but can slow down
2152 parser construction in stable code. So Parse::RecDescent provides
2153 the <nocheck> directive to turn them off. The directive can only
2154 appear before the first rule definition, and switches off checking
2155 throughout the rest of the current grammar.
2156 Typically, this directive would be added when a parser has been
2158 thoroughly tested and is ready for release.
2159 Subrule argument lists
2161 It is occasionally useful to pass data to a subrule which is being
2162 invoked. For example, consider the following grammar fragment:
2163 classdecl: keyword decl
2165 keyword: 'struct' | 'class';
2167 decl: # WHATEVER
2169 The "decl" rule might wish to know which of the two keywords was used
2171 (since it may affect some aspect of the way the subsequent declaration
2172 is interpreted). "Parse::RecDescent" allows the grammar designer to
2173 pass data into a rule, by placing that data in an argument list (that
2174 is, in square brackets) immediately after any subrule item in a
2175 production. Hence, we could pass the keyword to "decl" as follows:
2176 classdecl: keyword decl[ $item[1] ]
2178 keyword: 'struct' | 'class';
2180 decl: # WHATEVER
2182 The argument list can consist of any number (including zero!) of comma-
2184 separated Perl expressions. In other words, it looks exactly like a
2185 Perl anonymous array reference. For example, we could pass the keyword,
2186 the name of the surrounding rule, and the literal 'keyword' to "decl"
2187 like so:
2188 classdecl: keyword decl[$item[1],$item[0],'keyword']
2190 keyword: 'struct' | 'class';
2192 decl: # WHATEVER
2194 Within the rule to which the data is passed ("decl" in the above
2196 examples) that data is available as the elements of a local variable
2197 @arg. Hence "decl" might report its intentions as follows:
2198 classdecl: keyword decl[$item[1],$item[0],'keyword']
2200 keyword: 'struct' | 'class';
2202 decl: { print "Declaring $arg[0] (a $arg[2])\n";
2204 print "(this rule called by $arg[1])" }
2205 Subrule argument lists can also be interpreted as hashes, simply by
2207 using the local variable %arg instead of @arg. Hence we could rewrite
2208 the previous example:
2209 classdecl: keyword decl[keyword => $item[1],
2211 caller => $item[0],
2212 type => 'keyword']
2213 keyword: 'struct' | 'class';
2215 decl: { print "Declaring $arg{keyword} (a $arg{type})\n";
2217 print "(this rule called by $arg{caller})" }
2218 Both @arg and %arg are always available, so the grammar designer may
2220 choose whichever convention (or combination of conventions) suits best.
2221 Subrule argument lists are also useful for creating "rule templates"
2223 (especially when used in conjunction with the "<matchrule:...>"
2224 directive). For example, the subrule:
2225 list: <matchrule:$arg{rule}> /$arg{sep}/ list[%arg]
2227 { $return = [ $item[1], @{$item[3]} ] }
2228 | <matchrule:$arg{rule}>
2229 { $return = [ $item[1]] }
2230 is a handy template for the common problem of matching a separated
2232 list. For example:
2233 function: 'func' name '(' list[rule=>'param',sep=>';'] ')'
2235 param: list[rule=>'name',sep=>','] ':' typename
2237 name: /\w+/
2239 typename: name
2241 When a subrule argument list is used with a repeated subrule, the
2243 argument list goes before the repetition specifier:
2244 list: /some|many/ thing[ $item[1] ](s)
2246 The argument list is "late bound". That is, it is re-evaluated for
2248 every repetition of the repeated subrule. This means that each
2249 repeated attempt to match the subrule may be passed a completely
2250 different set of arguments if the value of the expression in the
2251 argument list changes between attempts. So, for example, the grammar:
2252 { $::species = 'dogs' }
2254 pair: 'two' animal[$::species](s)
2256 animal: /$arg[0]/ { $::species = 'cats' }
2258 will match the string "two dogs cats cats" completely, whereas it will
2260 only match the string "two dogs dogs dogs" up to the eighth letter. If
2261 the value of the argument list were "early bound" (that is, evaluated
2262 only the first time a repeated subrule match is attempted), one would
2263 expect the matching behaviours to be reversed.
2264 Of course, it is possible to effectively "early bind" such argument
2266 lists by passing them a value which does not change on each repetition.
2267 For example:
2268 { $::species = 'dogs' }
2270 pair: 'two' { $::species } animal[$item[2]](s)
2272 animal: /$arg[0]/ { $::species = 'cats' }
2274 Arguments can also be passed to the start rule, simply by appending
2276 them to the argument list with which the start rule is called (after
2277 the "line number" parameter). For example, given:
2278 $parser = new Parse::RecDescent ( $grammar );
2280 $parser->data($text, 1, "str", 2, \@arr);
2282 # ^^^^^ ^ ^^^^^^^^^^^^^^^
2284 # | | |
2285 # TEXT TO BE PARSED | |
2286 # STARTING LINE NUMBER |
2287 # ELEMENTS OF @arg WHICH IS PASSED TO RULE data
2288 then within the productions of the rule "data", the array @arg will
2290 contain "("str", 2, \@arr)".
2291 Alternations
2293 Alternations are implicit (unnamed) rules defined as part of a
2294 production. An alternation is defined as a series of '|'-separated
2295 productions inside a pair of round brackets. For example:
2296 character: 'the' ( good | bad | ugly ) /dude/
2298 Every alternation implicitly defines a new subrule, whose
2300 automatically-generated name indicates its origin:
2301 "_alternation_<I>_of_production_<P>_of_rule<R>" for the appropriate
2302 values of <I>, <P>, and <R>. A call to this implicit subrule is then
2303 inserted in place of the brackets. Hence the above example is merely a
2304 convenient short-hand for:
2305 character: 'the'
2307 _alternation_1_of_production_1_of_rule_character
2308 /dude/
2309 _alternation_1_of_production_1_of_rule_character:
2311 good | bad | ugly
2312 Since alternations are parsed by recursively calling the parser
2314 generator, any type(s) of item can appear in an alternation. For
2315 example:
2316 character: 'the' ( 'high' "plains" # Silent, with poncho
2318 | /no[- ]name/ # Silent, no poncho
2319 | vengeance_seeking # Poncho-optional
2320 | <error>
2321 ) drifter
2322 In this case, if an error occurred, the automatically generated message
2324 would be:
2325 ERROR (line <N>): Invalid implicit subrule: Expected
2327 'high' or /no[- ]name/ or generic,
2328 but found "pacifist" instead
2329 Since every alternation actually has a name, it's even possible to
2331 extend or replace them:
2332 parser->Replace(
2334 "_alternation_1_of_production_1_of_rule_character:
2335 'generic Eastwood'"
2336 );
2337 More importantly, since alternations are a form of subrule, they can be
2339 given repetition specifiers:
2340 character: 'the' ( good | bad | ugly )(?) /dude/
2342 Incremental Parsing
2344 "Parse::RecDescent" provides two methods - "Extend" and "Replace" -
2345 which can be used to alter the grammar matched by a parser. Both
2346 methods take the same argument as "Parse::RecDescent::new", namely a
2347 grammar specification string
2348 "Parse::RecDescent::Extend" interprets the grammar specification and
2350 adds any productions it finds to the end of the rules for which they
2351 are specified. For example:
2352 $add = "name: 'Jimmy-Bob' | 'Bobby-Jim'\ndesc: colour /necks?/";
2354 parser->Extend($add);
2355 adds two productions to the rule "name" (creating it if necessary) and
2357 one production to the rule "desc".
2358 "Parse::RecDescent::Replace" is identical, except that it first resets
2360 are rule specified in the additional grammar, removing any existing
2361 productions. Hence after:
2362 $add = "name: 'Jimmy-Bob' | 'Bobby-Jim'\ndesc: colour /necks?/";
2364 parser->Replace($add);
2365 there are only valid "name"s and the one possible description.
2367 A more interesting use of the "Extend" and "Replace" methods is to call
2369 them inside the action of an executing parser. For example:
2370 typedef: 'typedef' type_name identifier ';'
2372 { $thisparser->Extend("type_name: '$item[3]'") }
2373 | <error>
2374 identifier: ...!type_name /[A-Za-z_]w*/
2376 which automatically prevents type names from being typedef'd, or:
2378 command: 'map' key_name 'to' abort_key
2380 { $thisparser->Replace("abort_key: '$item[2]'") }
2381 | 'map' key_name 'to' key_name
2382 { map_key($item[2],$item[4]) }
2383 | abort_key
2384 { exit if confirm("abort?") }
2385 abort_key: 'q'
2387 key_name: ...!abort_key /[A-Za-z]/
2389 which allows the user to change the abort key binding, but not to
2391 unbind it.
2392 The careful use of such constructs makes it possible to reconfigure a a
2394 running parser, eliminating the need for semantic feedback by providing
2395 syntactic feedback instead. However, as currently implemented,
2396 "Replace()" and "Extend()" have to regenerate and re-"eval" the entire
2397 parser whenever they are called. This makes them quite slow for large
2398 grammars.
2399 In such cases, the judicious use of an interpolated regex is likely to
2401 be far more efficient:
2402 typedef: 'typedef' type_name/ identifier ';'
2404 { $thisparser->{local}{type_name} .= "|$item[3]" }
2405 | <error>
2406 identifier: ...!type_name /[A-Za-z_]w*/
2408 type_name: /$thisparser->{local}{type_name}/
2410 Precompiling parsers
2412 Normally Parse::RecDescent builds a parser from a grammar at run-time.
2413 That approach simplifies the design and implementation of parsing code,
2414 but has the disadvantage that it slows the parsing process down - you
2415 have to wait for Parse::RecDescent to build the parser every time the
2416 program runs. Long or complex grammars can be particularly slow to
2417 build, leading to unacceptable delays at start-up.
2418 To overcome this, the module provides a way of "pre-building" a parser
2420 object and saving it in a separate module. That module can then be used
2421 to create clones of the original parser.
2422 A grammar may be precompiled using the "Precompile" class method. For
2424 example, to precompile a grammar stored in the scalar $grammar, and
2425 produce a class named PreGrammar in a module file named PreGrammar.pm,
2426 you could use:
2427 use Parse::RecDescent;
2429 Parse::RecDescent->Precompile([$options_hashref], $grammar, "PreGrammar", ["RuntimeClass"]);
2431 The first required argument is the grammar string, the second is the
2433 name of the class to be built. The name of the module file is generated
2434 automatically by appending ".pm" to the last element of the class name.
2435 Thus
2436 Parse::RecDescent->Precompile($grammar, "My::New::Parser");
2438 would produce a module file named Parser.pm.
2440 After the class name, you may specify the name of the runtime_class
2442 called by the Precompiled parser. See "Precompiled runtimes" for more
2443 details.
2444 An optional hash reference may be supplied as the first argument to
2446 "Precompile". This argument is currently EXPERIMENTAL, and may change
2447 in a future release of Parse::RecDescent. The only supported option is
2448 currently "-standalone", see "Standalone precompiled parsers".
2449 It is somewhat tedious to have to write a small Perl program just to
2451 generate a precompiled grammar class, so Parse::RecDescent has some
2452 special magic that allows you to do the job directly from the command-
2453 line.
2454 If your grammar is specified in a file named grammar, you can generate
2456 a class named Yet::Another::Grammar like so:
2457 > perl -MParse::RecDescent - grammar Yet::Another::Grammar [Runtime::Class]
2459 This would produce a file named Grammar.pm containing the full
2461 definition of a class called Yet::Another::Grammar. Of course, to use
2462 that class, you would need to put the Grammar.pm file in a directory
2463 named Yet/Another, somewhere in your Perl include path.
2464 Having created the new class, it's very easy to use it to build a
2466 parser. You simply "use" the new module, and then call its "new" method
2467 to create a parser object. For example:
2468 use Yet::Another::Grammar;
2470 my $parser = Yet::Another::Grammar->new();
2471 The effect of these two lines is exactly the same as:
2473 use Parse::RecDescent;
2475 open GRAMMAR_FILE, "grammar" or die;
2477 local $/;
2478 my $grammar = <GRAMMAR_FILE>;
2479 my $parser = Parse::RecDescent->new($grammar);
2481 only considerably faster.
2483 Note however that the parsers produced by either approach are exactly
2485 the same, so whilst precompilation has an effect on set-up speed, it
2486 has no effect on parsing speed. RecDescent 2.0 will address that
2487 problem.
2488 Standalone precompiled parsers
2490 Until version 1.967003 of Parse::RecDescent, parser modules built with
2492 "Precompile" were dependent on Parse::RecDescent. Future
2493 Parse::RecDescent releases with different internal implementations
2494 would break pre-existing precompiled parsers.
2495 Version 1.967_005 added the ability for Parse::RecDescent to include
2497 itself in the resulting .pm file if you pass the boolean option
2498 "-standalone" to "Precompile":
2499 Parse::RecDescent->Precompile({ -standalone => 1, },
2501 $grammar, "My::New::Parser");
2502 Parse::RecDescent is included as $class::_Runtime in order to avoid
2504 conflicts between an installed version of Parse::RecDescent and other
2505 precompiled, standalone parser made with Parse::RecDescent. The name
2506 of this class may be changed with the "-runtime_class" option to
2507 Precompile. This renaming is experimental, and is subject to change in
2508 future versions.
2509 Precompiled parsers remain dependent on Parse::RecDescent by default,
2511 as this feature is still considered experimental. In the future,
2512 standalone parsers will become the default.
2513 Precompiled runtimes
2515 Standalone precompiled parsers each include a copy of
2517 Parse::RecDescent. For users who have a family of related precompiled
2518 parsers, this is very inefficient. "Precompile" now supports an
2519 experimental "-runtime_class" option. To build a precompiled parser
2520 with a different runtime name, call:
2521 Parse::RecDescent->Precompile({
2523 -standalone => 1,
2524 -runtime_class => "My::Runtime",
2525 },
2526 $grammar, "My::New::Parser");
2527 The resulting standalone parser will contain a copy of
2529 Parse::RecDescent, renamed to "My::Runtime".
2530 To build a set of parsers that "use" a custom-named runtime, without
2532 including that runtime in the output, simply build those parsers with
2533 "-runtime_class" and without "-standalone":
2534 Parse::RecDescent->Precompile({
2536 -runtime_class => "My::Runtime",
2537 },
2538 $grammar, "My::New::Parser");
2539 The runtime itself must be generated as well, so that it may be "use"d
2541 by My::New::Parser. To generate the runtime file, use one of the two
2542 folling calls:
2543 Parse::RecDescent->PrecompiledRuntime("My::Runtime");
2545 Parse::RecDescent->Precompile({
2547 -standalone => 1,
2548 -runtime_class => "My::Runtime",
2549 },
2550 '', # empty grammar
2551 "My::Runtime");
2552
2554 This section describes common mistakes that grammar writers seem to
2555 make on a regular basis.
2556 1. Expecting an error to always invalidate a parse
2558 A common mistake when using error messages is to write the grammar like
2559 this:
2560 file: line(s)
2562 line: line_type_1
2564 | line_type_2
2565 | line_type_3
2566 | <error>
2567 The expectation seems to be that any line that is not of type 1, 2 or 3
2569 will invoke the "<error>" directive and thereby cause the parse to
2570 fail.
2571 Unfortunately, that only happens if the error occurs in the very first
2573 line. The first rule states that a "file" is matched by one or more
2574 lines, so if even a single line succeeds, the first rule is completely
2575 satisfied and the parse as a whole succeeds. That means that any error
2576 messages generated by subsequent failures in the "line" rule are
2577 quietly ignored.
2578 Typically what's really needed is this:
2580 file: line(s) eofile { $return = $item[1] }
2582 line: line_type_1
2584 | line_type_2
2585 | line_type_3
2586 | <error>
2587 eofile: /^\Z/
2589 The addition of the "eofile" subrule to the first production means
2591 that a file only matches a series of successful "line" matches that
2592 consume the complete input text. If any input text remains after the
2593 lines are matched, there must have been an error in the last "line". In
2594 that case the "eofile" rule will fail, causing the entire "file" rule
2595 to fail too.
2596 Note too that "eofile" must match "/^\Z/" (end-of-text), not "/^\cZ/"
2598 or "/^\cD/" (end-of-file).
2599 And don't forget the action at the end of the production. If you just
2601 write:
2602 file: line(s) eofile
2604 then the value returned by the "file" rule will be the value of its
2606 last item: "eofile". Since "eofile" always returns an empty string on
2607 success, that will cause the "file" rule to return that empty string.
2608 Apart from returning the wrong value, returning an empty string will
2609 trip up code such as:
2610 $parser->file($filetext) || die;
2612 (since "" is false).
2614 Remember that Parse::RecDescent returns undef on failure, so the only
2616 safe test for failure is:
2617 defined($parser->file($filetext)) || die;
2619 2. Using a "return" in an action
2621 An action is like a "do" block inside the subroutine implementing the
2622 surrounding rule. So if you put a "return" statement in an action:
2623 range: '(' start '..' end )'
2625 { return $item{end} }
2626 /\s+/
2627 that subroutine will immediately return, without checking the rest of
2629 the items in the current production (e.g. the "/\s+/") and without
2630 setting up the necessary data structures to tell the parser that the
2631 rule has succeeded.
2632 The correct way to set a return value in an action is to set the
2634 $return variable:
2635 range: '(' start '..' end )'
2637 { $return = $item{end} }
2638 /\s+/
2639 2. Setting $Parse::RecDescent::skip at parse time
2641 If you want to change the default skipping behaviour (see "Terminal
2642 Separators" and the "<skip:...>" directive) by setting
2643 $Parse::RecDescent::skip you have to remember to set this variable
2644 before creating the grammar object.
2645 For example, you might want to skip all Perl-like comments with this
2647 regular expression:
2648 my $skip_spaces_and_comments = qr/
2650 (?mxs:
2651 \s+ # either spaces
2652 | \# .*?$ # or a dash and whatever up to the end of line
2653 )* # repeated at will (in whatever order)
2654 /;
2655 And then:
2657 my $parser1 = Parse::RecDescent->new($grammar);
2659 $Parse::RecDescent::skip = $skip_spaces_and_comments;
2661 my $parser2 = Parse::RecDescent->new($grammar);
2663 $parser1->parse($text); # this does not cope with comments
2665 $parser2->parse($text); # this skips comments correctly
2666 The two parsers behave differently, because any skipping behaviour
2668 specified via $Parse::RecDescent::skip is hard-coded when the grammar
2669 object is built, not at parse time.
2670
2672 Diagnostics are intended to be self-explanatory (particularly if you
2673 use -RD_HINT (under perl -s) or define $::RD_HINT inside the program).
2674 "Parse::RecDescent" currently diagnoses the following:
2676 · Invalid regular expressions used as pattern terminals (fatal
2678 error).
2679 · Invalid Perl code in code blocks (fatal error).
2681 · Lookahead used in the wrong place or in a nonsensical way (fatal
2683 error).
2684 · "Obvious" cases of left-recursion (fatal error).
2686 · Missing or extra components in a "<leftop>" or "<rightop>"
2688 directive.
2689 · Unrecognisable components in the grammar specification (fatal
2691 error).
2692 · "Orphaned" rule components specified before the first rule (fatal
2694 error) or after an "<error>" directive (level 3 warning).
2695 · Missing rule definitions (this only generates a level 3 warning,
2697 since you may be providing them later via
2698 "Parse::RecDescent::Extend()").
2699 · Instances where greedy repetition behaviour will almost certainly
2701 cause the failure of a production (a level 3 warning - see "ON-
2702 GOING ISSUES AND FUTURE DIRECTIONS" below).
2703 · Attempts to define rules named 'Replace' or 'Extend', which cannot
2705 be called directly through the parser object because of the
2706 predefined meaning of "Parse::RecDescent::Replace" and
2707 "Parse::RecDescent::Extend". (Only a level 2 warning is generated,
2708 since such rules can still be used as subrules).
2709 · Productions which consist of a single "<error?>" directive, and
2711 which therefore may succeed unexpectedly (a level 2 warning, since
2712 this might conceivably be the desired effect).
2713 · Multiple consecutive lookahead specifiers (a level 1 warning only,
2715 since their effects simply accumulate).
2716 · Productions which start with a "<reject>" or "<rulevar:...>"
2718 directive. Such productions are optimized away (a level 1 warning).
2719 · Rules which are autogenerated under $::AUTOSTUB (a level 1
2721 warning).
2722
2724 Damian Conway (damian@conway.org) Jeremy T. Braun (JTBRAUN@CPAN.org)
2725 [current maintainer]
2726
2728 There are undoubtedly serious bugs lurking somewhere in this much code
2729 :-) Bug reports, test cases and other feedback are most welcome.
2730 Ongoing annoyances include:
2732 · There's no support for parsing directly from an input stream. If
2734 and when the Perl Gods give us regular expressions on streams, this
2735 should be trivial (ahem!) to implement.
2736 · The parser generator can get confused if actions aren't properly
2738 closed or if they contain particularly nasty Perl syntax errors
2739 (especially unmatched curly brackets).
2740 · The generator only detects the most obvious form of left recursion
2742 (potential recursion on the first subrule in a rule). More subtle
2743 forms of left recursion (for example, through the second item in a
2744 rule after a "zero" match of a preceding "zero-or-more" repetition,
2745 or after a match of a subrule with an empty production) are not
2746 found.
2747 · Instead of complaining about left-recursion, the generator should
2749 silently transform the grammar to remove it. Don't expect this
2750 feature any time soon as it would require a more sophisticated
2751 approach to parser generation than is currently used.
2752 · The generated parsers don't always run as fast as might be wished.
2754 · The meta-parser should be bootstrapped using "Parse::RecDescent"
2756 :-)
2757
2759 1. Repetitions are "incorrigibly greedy" in that they will eat
2760 everything they can and won't backtrack if that behaviour causes a
2761 production to fail needlessly. So, for example:
2762 rule: subrule(s) subrule
2764 will never succeed, because the repetition will eat all the
2766 subrules it finds, leaving none to match the second item. Such
2767 constructions are relatively rare (and "Parse::RecDescent::new"
2768 generates a warning whenever they occur) so this may not be a
2769 problem, especially since the insatiable behaviour can be overcome
2770 "manually" by writing:
2771 rule: penultimate_subrule(s) subrule
2773 penultimate_subrule: subrule ...subrule
2775 The issue is that this construction is exactly twice as expensive
2777 as the original, whereas backtracking would add only 1/N to the
2778 cost (for matching N repetitions of "subrule"). I would welcome
2779 feedback on the need for backtracking; particularly on cases where
2780 the lack of it makes parsing performance problematical.
2781 2. Having opened that can of worms, it's also necessary to consider
2783 whether there is a need for non-greedy repetition specifiers.
2784 Again, it's possible (at some cost) to manually provide the
2785 required functionality:
2786 rule: nongreedy_subrule(s) othersubrule
2788 nongreedy_subrule: subrule ...!othersubrule
2790 Overall, the issue is whether the benefit of this extra
2792 functionality outweighs the drawbacks of further complicating the
2793 (currently minimalist) grammar specification syntax, and (worse)
2794 introducing more overhead into the generated parsers.
2795 3. An "<autocommit>" directive would be nice. That is, it would be
2797 useful to be able to say:
2798 command: <autocommit>
2800 command: 'find' name
2801 | 'find' address
2802 | 'do' command 'at' time 'if' condition
2803 | 'do' command 'at' time
2804 | 'do' command
2805 | unusual_command
2806 and have the generator work out that this should be "pruned" thus:
2808 command: 'find' name
2810 | 'find' <commit> address
2811 | 'do' <commit> command <uncommit>
2812 'at' time
2813 'if' <commit> condition
2814 | 'do' <commit> command <uncommit>
2815 'at' <commit> time
2816 | 'do' <commit> command
2817 | unusual_command
2818 There are several issues here. Firstly, should the "<autocommit>"
2820 automatically install an "<uncommit>" at the start of the last
2821 production (on the grounds that the "command" rule doesn't know
2822 whether an "unusual_command" might start with "find" or "do") or
2823 should the "unusual_command" subgraph be analysed (to see if it
2824 might be viable after a "find" or "do")?
2825 The second issue is how regular expressions should be treated. The
2827 simplest approach would be simply to uncommit before them (on the
2828 grounds that they might match). Better efficiency would be obtained
2829 by analyzing all preceding literal tokens to determine whether the
2830 pattern would match them.
2831 Overall, the issues are: can such automated "pruning" approach a
2833 hand-tuned version sufficiently closely to warrant the extra set-up
2834 expense, and (more importantly) is the problem important enough to
2835 even warrant the non-trivial effort of building an automated
2836 solution?
2837
2839 Source Code Repository
2840 <http://github.com/jtbraun/Parse-RecDescent>
2841 Mailing List
2843 Visit <http://www.perlfoundation.org/perl5/index.cgi?parse_recdescent>
2844 to sign up for the mailing list.
2845 <http://www.PerlMonks.org> is also a good place to ask questions.
2847 Previous posts about Parse::RecDescent can typically be found with this
2848 search: <http://perlmonks.org/index.pl?node=recdescent>.
2849 FAQ
2851 Visit Parse::RecDescent::FAQ for answers to frequently (and not so
2852 frequently) asked questions about Parse::RecDescent.
2853 View/Report Bugs
2855 To view the current bug list or report a new issue visit
2856 <https://rt.cpan.org/Public/Dist/Display.html?Name=Parse-RecDescent>.
2857
2859 Regexp::Grammars provides Parse::RecDescent style parsing using native
2860 Perl 5.10 regular expressions.
2861
2863 Copyright (c) 1997-2007, Damian Conway "<DCONWAY@CPAN.org>". All rights
2864 reserved.
2865 This module is free software; you can redistribute it and/or modify it
2867 under the same terms as Perl itself. See perlartistic.
2868
2870 BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
2871 FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT
2872 WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER
2873 PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND,
2874 EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
2875 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
2876 ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
2877 YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
2878 NECESSARY SERVICING, REPAIR, OR CORRECTION.
2879 IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
2881 WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
2882 REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE
2883 TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR
2884 CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE
2885 SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
2886 RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
2887 FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
2888 SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
2889 DAMAGES.
2890perl v5.28.0 2017-04-04 Parse::RecDescent(3)