1PERLDATA(1) Perl Programmers Reference Guide PERLDATA(1)
2
6 perldata - Perl data types
7
9 Variable names
10 Perl has three built-in data types: scalars, arrays of scalars, and
11 associative arrays of scalars, known as "hashes". A scalar is a single
12 string (of any size, limited only by the available memory), number, or
13 a reference to something (which will be discussed in perlref). Normal
14 arrays are ordered lists of scalars indexed by number, starting with 0.
15 Hashes are unordered collections of scalar values indexed by their
16 associated string key.
17 Values are usually referred to by name, or through a named reference.
19 The first character of the name tells you to what sort of data
20 structure it refers. The rest of the name tells you the particular
21 value to which it refers. Usually this name is a single identifier,
22 that is, a string beginning with a letter or underscore, and containing
23 letters, underscores, and digits. In some cases, it may be a chain of
24 identifiers, separated by "::" (or by the deprecated "'"); all but the
25 last are interpreted as names of packages, to locate the namespace in
26 which to look up the final identifier (see "Packages" in perlmod for
27 details). For a more in-depth discussion on identifiers, see
28 "Identifier parsing". It's possible to substitute for a simple
29 identifier, an expression that produces a reference to the value at
30 runtime. This is described in more detail below and in perlref.
31 Perl also has its own built-in variables whose names don't follow these
33 rules. They have strange names so they don't accidentally collide with
34 one of your normal variables. Strings that match parenthesized parts
35 of a regular expression are saved under names containing only digits
36 after the "$" (see perlop and perlre). In addition, several special
37 variables that provide windows into the inner working of Perl have
38 names containing punctuation characters. These are documented in
39 perlvar.
40 Scalar values are always named with '$', even when referring to a
42 scalar that is part of an array or a hash. The '$' symbol works
43 semantically like the English word "the" in that it indicates a single
44 value is expected.
45 $days # the simple scalar value "days"
47 $days[28] # the 29th element of array @days
48 $days{'Feb'} # the 'Feb' value from hash %days
49 $#days # the last index of array @days
50 Entire arrays (and slices of arrays and hashes) are denoted by '@',
52 which works much as the word "these" or "those" does in English, in
53 that it indicates multiple values are expected.
54 @days # ($days[0], $days[1],... $days[n])
56 @days[3,4,5] # same as ($days[3],$days[4],$days[5])
57 @days{'a','c'} # same as ($days{'a'},$days{'c'})
58 Entire hashes are denoted by '%':
60 %days # (key1, val1, key2, val2 ...)
62 In addition, subroutines are named with an initial '&', though this is
64 optional when unambiguous, just as the word "do" is often redundant in
65 English. Symbol table entries can be named with an initial '*', but
66 you don't really care about that yet (if ever :-).
67 Every variable type has its own namespace, as do several non-variable
69 identifiers. This means that you can, without fear of conflict, use
70 the same name for a scalar variable, an array, or a hash--or, for that
71 matter, for a filehandle, a directory handle, a subroutine name, a
72 format name, or a label. This means that $foo and @foo are two
73 different variables. It also means that $foo[1] is a part of @foo, not
74 a part of $foo. This may seem a bit weird, but that's okay, because it
75 is weird.
76 Because variable references always start with '$', '@', or '%', the
78 "reserved" words aren't in fact reserved with respect to variable
79 names. They are reserved with respect to labels and filehandles,
80 however, which don't have an initial special character. You can't have
81 a filehandle named "log", for instance. Hint: you could say
82 "open(LOG,'logfile')" rather than "open(log,'logfile')". Using
83 uppercase filehandles also improves readability and protects you from
84 conflict with future reserved words. Case is significant--"FOO",
85 "Foo", and "foo" are all different names. Names that start with a
86 letter or underscore may also contain digits and underscores.
87 It is possible to replace such an alphanumeric name with an expression
89 that returns a reference to the appropriate type. For a description of
90 this, see perlref.
91 Names that start with a digit may contain only more digits. Names that
93 do not start with a letter, underscore, digit or a caret are limited to
94 one character, e.g., $% or $$. (Most of these one character names
95 have a predefined significance to Perl. For instance, $$ is the
96 current process id. And all such names are reserved for Perl's
97 possible use.)
98 Identifier parsing
100 Up until Perl 5.18, the actual rules of what a valid identifier was
101 were a bit fuzzy. However, in general, anything defined here should
102 work on previous versions of Perl, while the opposite -- edge cases
103 that work in previous versions, but aren't defined here -- probably
104 won't work on newer versions. As an important side note, please note
105 that the following only applies to bareword identifiers as found in
106 Perl source code, not identifiers introduced through symbolic
107 references, which have much fewer restrictions. If working under the
108 effect of the "use utf8;" pragma, the following rules apply:
109 / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
111 (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
112 That is, a "start" character followed by any number of "continue"
114 characters. Perl requires every character in an identifier to also
115 match "\w" (this prevents some problematic cases); and Perl
116 additionally accepts identifier names beginning with an underscore.
117 If not under "use utf8", the source is treated as ASCII + 128 extra
119 generic characters, and identifiers should match
120 / (?aa) (?!\d) \w+ /x
122 That is, any word character in the ASCII range, as long as the first
124 character is not a digit.
125 There are two package separators in Perl: A double colon ("::") and a
127 single quote ("'"). Use of "'" as the package separator is deprecated
128 and will be removed in Perl 5.40. Normal identifiers can start or end
129 with a double colon, and can contain several parts delimited by double
130 colons. Single quotes have similar rules, but with the exception that
131 they are not legal at the end of an identifier: That is, "$'foo" and
132 "$foo'bar" are legal, but "$foo'bar'" is not.
133 Additionally, if the identifier is preceded by a sigil -- that is, if
135 the identifier is part of a variable name -- it may optionally be
136 enclosed in braces.
137 While you can mix double colons with singles quotes, the quotes must
139 come after the colons: "$::::'foo" and "$foo::'bar" are legal, but
140 "$::'::foo" and "$foo'::bar" are not.
141 Put together, a grammar to match a basic identifier becomes
143 /
145 (?(DEFINE)
146 (?<variable>
147 (?&sigil)
148 (?:
149 (?&normal_identifier)
150 | \{ \s* (?&normal_identifier) \s* \}
151 )
152 )
153 (?<normal_identifier>
154 (?: :: )* '?
155 (?&basic_identifier)
156 (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
157 (?: :: )*
158 )
159 (?<basic_identifier>
160 # is use utf8 on?
161 (?(?{ (caller(0))[8] & $utf8::hint_bits })
162 (?&Perl_XIDS) (?&Perl_XIDC)*
163 | (?aa) (?!\d) \w+
164 )
165 )
166 (?<sigil> [&*\$\@\%])
167 (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
168 (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
169 )
170 /x
171 Meanwhile, special identifiers don't follow the above rules; For the
173 most part, all of the identifiers in this category have a special
174 meaning given by Perl. Because they have special parsing rules, these
175 generally can't be fully-qualified. They come in six forms (but don't
176 use forms 5 and 6):
177 1. A sigil, followed solely by digits matching "\p{POSIX_Digit}", like
179 $0, $1, or $10000.
180 2. A sigil followed by a single character matching the
182 "\p{POSIX_Punct}" property, like $! or "%+", except the character
183 "{" doesn't work.
184 3. A sigil, followed by a caret and any one of the characters
186 "[][A-Z^_?\]", like $^V or $^].
187 4. Similar to the above, a sigil, followed by bareword text in braces,
189 where the first character is a caret. The next character is any
190 one of the characters "[][A-Z^_?\]", followed by ASCII word
191 characters. An example is "${^GLOBAL_PHASE}".
192 5. A sigil, followed by any single character in the range
194 "[\xA1-\xAC\xAE-\xFF]" when not under "use utf8". (Under
195 "use utf8", the normal identifier rules given earlier in this
196 section apply.) Use of non-graphic characters (the C1 controls,
197 the NO-BREAK SPACE, and the SOFT HYPHEN) has been disallowed since
198 v5.26.0. The use of the other characters is unwise, as these are
199 all reserved to have special meaning to Perl, and none of them
200 currently do have special meaning, though this could change without
201 notice.
202 Note that an implication of this form is that there are identifiers
204 only legal under "use utf8", and vice-versa, for example the
205 identifier "$état" is legal under "use utf8", but is otherwise
206 considered to be the single character variable $é followed by the
207 bareword "tat", the combination of which is a syntax error.
208 6. This is a combination of the previous two forms. It is valid only
210 when not under "use utf8" (normal identifier rules apply when under
211 "use utf8"). The form is a sigil, followed by text in braces,
212 where the first character is any one of the characters in the range
213 "[\x80-\xFF]" followed by ASCII word characters up to the trailing
214 brace.
215 The same caveats as the previous form apply: The non-graphic
217 characters are no longer allowed with "use utf8", it is unwise to
218 use this form at all, and utf8ness makes a big difference.
219 Prior to Perl v5.24, non-graphical ASCII control characters were also
221 allowed in some situations; this had been deprecated since v5.20.
222 Context
224 The interpretation of operations and values in Perl sometimes depends
225 on the requirements of the context around the operation or value.
226 There are two major contexts: list and scalar. Certain operations
227 return list values in contexts wanting a list, and scalar values
228 otherwise. If this is true of an operation it will be mentioned in the
229 documentation for that operation. In other words, Perl overloads
230 certain operations based on whether the expected return value is
231 singular or plural. Some words in English work this way, like "fish"
232 and "sheep".
233 In a reciprocal fashion, an operation provides either a scalar or a
235 list context to each of its arguments. For example, if you say
236 int( <STDIN> )
238 the integer operation provides scalar context for the <> operator,
240 which responds by reading one line from STDIN and passing it back to
241 the integer operation, which will then find the integer value of that
242 line and return that. If, on the other hand, you say
243 sort( <STDIN> )
245 then the sort operation provides list context for <>, which will
247 proceed to read every line available up to the end of file, and pass
248 that list of lines back to the sort routine, which will then sort those
249 lines and return them as a list to whatever the context of the sort
250 was.
251 Assignment is a little bit special in that it uses its left argument to
253 determine the context for the right argument. Assignment to a scalar
254 evaluates the right-hand side in scalar context, while assignment to an
255 array or hash evaluates the righthand side in list context. Assignment
256 to a list (or slice, which is just a list anyway) also evaluates the
257 right-hand side in list context.
258 When you use the "use warnings" pragma or Perl's -w command-line
260 option, you may see warnings about useless uses of constants or
261 functions in "void context". Void context just means the value has
262 been discarded, such as a statement containing only ""fred";" or
263 "getpwuid(0);". It still counts as scalar context for functions that
264 care whether or not they're being called in list context.
265 User-defined subroutines may choose to care whether they are being
267 called in a void, scalar, or list context. Most subroutines do not
268 need to bother, though. That's because both scalars and lists are
269 automatically interpolated into lists. See "wantarray" in perlfunc for
270 how you would dynamically discern your function's calling context.
271 Scalar values
273 All data in Perl is a scalar, an array of scalars, or a hash of
274 scalars. A scalar may contain one single value in any of three
275 different flavors: a number, a string, or a reference. In general,
276 conversion from one form to another is transparent. Although a scalar
277 may not directly hold multiple values, it may contain a reference to an
278 array or hash which in turn contains multiple values.
279 Scalars aren't necessarily one thing or another. There's no place to
281 declare a scalar variable to be of type "string", type "number", type
282 "reference", or anything else. Because of the automatic conversion of
283 scalars, operations that return scalars don't need to care (and in
284 fact, cannot care) whether their caller is looking for a string, a
285 number, or a reference. Perl is a contextually polymorphic language
286 whose scalars can be strings, numbers, or references (which includes
287 objects). Although strings and numbers are considered pretty much the
288 same thing for nearly all purposes, references are strongly-typed,
289 uncastable pointers with builtin reference-counting and destructor
290 invocation.
291 A scalar value is interpreted as FALSE in the Boolean sense if it is
293 undefined, the null string or the number 0 (or its string equivalent,
294 "0"), and TRUE if it is anything else. The Boolean context is just a
295 special kind of scalar context where no conversion to a string or a
296 number is ever performed. Negation of a true value by "!" or "not"
297 returns a special false value. When evaluated as a string it is
298 treated as "", but as a number, it is treated as 0. Most Perl
299 operators that return true or false behave this way.
300 There are actually two varieties of null strings (sometimes referred to
302 as "empty" strings), a defined one and an undefined one. The defined
303 version is just a string of length zero, such as "". The undefined
304 version is the value that indicates that there is no real value for
305 something, such as when there was an error, or at end of file, or when
306 you refer to an uninitialized variable or element of an array or hash.
307 Although in early versions of Perl, an undefined scalar could become
308 defined when first used in a place expecting a defined value, this no
309 longer happens except for rare cases of autovivification as explained
310 in perlref. You can use the defined() operator to determine whether a
311 scalar value is defined (this has no meaning on arrays or hashes), and
312 the undef() operator to produce an undefined value.
313 To find out whether a given string is a valid non-zero number, it's
315 sometimes enough to test it against both numeric 0 and also lexical "0"
316 (although this will cause noises if warnings are on). That's because
317 strings that aren't numbers count as 0, just as they do in awk:
318 if ($str == 0 && $str ne "0") {
320 warn "That doesn't look like a number";
321 }
322 That method may be best because otherwise you won't treat IEEE
324 notations like "NaN" or "Infinity" properly. At other times, you might
325 prefer to determine whether string data can be used numerically by
326 calling the POSIX::strtod() function or by inspecting your string with
327 a regular expression (as documented in perlre).
328 warn "has nondigits" if /\D/;
330 warn "not a natural number" unless /^\d+$/; # rejects -3
331 warn "not an integer" unless /^-?\d+$/; # rejects +3
332 warn "not an integer" unless /^[+-]?\d+$/;
333 warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
334 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
335 warn "not a C float"
336 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
337 The length of an array is a scalar value. You may find the length of
339 array @days by evaluating $#days, as in csh. However, this isn't the
340 length of the array; it's the subscript of the last element, which is a
341 different value since there is ordinarily a 0th element. Assigning to
342 $#days actually changes the length of the array. Shortening an array
343 this way destroys intervening values. Lengthening an array that was
344 previously shortened does not recover values that were in those
345 elements.
346 You can also gain some minuscule measure of efficiency by pre-extending
348 an array that is going to get big. You can also extend an array by
349 assigning to an element that is off the end of the array. You can
350 truncate an array down to nothing by assigning the null list () to it.
351 The following are equivalent:
352 @whatever = ();
354 $#whatever = -1;
355 If you evaluate an array in scalar context, it returns the length of
357 the array. (Note that this is not true of lists, which return the last
358 value, like the C comma operator, nor of built-in functions, which
359 return whatever they feel like returning.) The following is always
360 true:
361 scalar(@whatever) == $#whatever + 1;
363 Some programmers choose to use an explicit conversion so as to leave
365 nothing to doubt:
366 $element_count = scalar(@whatever);
368 If you evaluate a hash in scalar context, it returns a false value if
370 the hash is empty. If there are any key/value pairs, it returns a true
371 value. A more precise definition is version dependent.
372 Prior to Perl 5.25 the value returned was a string consisting of the
374 number of used buckets and the number of allocated buckets, separated
375 by a slash. This is pretty much useful only to find out whether Perl's
376 internal hashing algorithm is performing poorly on your data set. For
377 example, you stick 10,000 things in a hash, but evaluating %HASH in
378 scalar context reveals "1/16", which means only one out of sixteen
379 buckets has been touched, and presumably contains all 10,000 of your
380 items. This isn't supposed to happen.
381 As of Perl 5.25 the return was changed to be the count of keys in the
383 hash. If you need access to the old behavior you can use
384 Hash::Util::bucket_ratio() instead.
385 If a tied hash is evaluated in scalar context, the "SCALAR" method is
387 called (with a fallback to "FIRSTKEY").
388 You can preallocate space for a hash by assigning to the keys()
390 function. This rounds up the allocated buckets to the next power of
391 two:
392 keys(%users) = 1000; # allocate 1024 buckets
394 Scalar value constructors
396 Numeric literals are specified in any of the following floating point
397 or integer formats:
398 12345
400 12345.67
401 .23E-10 # a very small number
402 3.14_15_92 # a very important number
403 4_294_967_296 # underscore for legibility
404 0xff # hex
405 0xdead_beef # more hex
406 0377 # octal (only numbers, begins with 0)
407 0o12_345 # alternative octal (introduced in Perl 5.33.5)
408 0b011011 # binary
409 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
410 You are allowed to use underscores (underbars) in numeric literals
412 between digits for legibility (but not multiple underscores in a row:
413 "23__500" is not legal; "23_500" is). You could, for example, group
414 binary digits by threes (as for a Unix-style mode argument such as
415 0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or
416 in other groups.
417 String literals are usually delimited by either single or double
419 quotes. They work much like quotes in the standard Unix shells:
420 double-quoted string literals are subject to backslash and variable
421 substitution; single-quoted strings are not (except for "\'" and "\\").
422 The usual C-style backslash rules apply for making characters such as
423 newline, tab, etc., as well as some more exotic forms. See "Quote and
424 Quote-like Operators" in perlop for a list.
425 Hexadecimal, octal, or binary, representations in string literals (e.g.
427 '0xff') are not automatically converted to their integer
428 representation. The hex() and oct() functions make these conversions
429 for you. See "hex" in perlfunc and "oct" in perlfunc for more details.
430 Hexadecimal floating point can start just like a hexadecimal literal,
432 and it can be followed by an optional fractional hexadecimal part, but
433 it must be followed by "p", an optional sign, and a power of two. The
434 format is useful for accurately presenting floating point values,
435 avoiding conversions to or from decimal floating point, and therefore
436 avoiding possible loss in precision. Notice that while most current
437 platforms use the 64-bit IEEE 754 floating point, not all do. Another
438 potential source of (low-order) differences are the floating point
439 rounding modes, which can differ between CPUs, operating systems, and
440 compilers, and which Perl doesn't control.
441 You can also embed newlines directly in your strings, i.e., they can
443 end on a different line than they begin. This is nice, but if you
444 forget your trailing quote, the error will not be reported until Perl
445 finds another line containing the quote character, which may be much
446 further on in the script. Variable substitution inside strings is
447 limited to scalar variables, arrays, and array or hash slices. (In
448 other words, names beginning with $ or @, followed by an optional
449 bracketed expression as a subscript.) The following code segment
450 prints out "The price is $100."
451 $Price = '$100'; # not interpolated
453 print "The price is $Price.\n"; # interpolated
454 There is no double interpolation in Perl, so the $100 is left as is.
456 By default floating point numbers substituted inside strings use the
458 dot (".") as the decimal separator. If "use locale" is in effect, and
459 POSIX::setlocale() has been called, the character used for the decimal
460 separator is affected by the LC_NUMERIC locale. See perllocale and
461 POSIX.
462 Demarcated variable names using braces
464 As in some shells, you can enclose the variable name in braces as a
466 demarcator to disambiguate it from following alphanumerics and
467 underscores or other text. You must also do this when interpolating a
468 variable into a string to separate the variable name from a following
469 double-colon or an apostrophe since these would be otherwise treated as
470 a package separator:
471 $who = "Larry";
473 print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
474 print "We use ${who}speak when ${who}'s here.\n";
475 Without the braces, Perl would have looked for a $whospeak, a $who::0,
477 and a "$who's" variable. The last two would be the $0 and the $s
478 variables in the (presumably) non-existent package "who".
479 In fact, a simple identifier within such curly braces is forced to be a
481 string, and likewise within a hash subscript. Neither need quoting. Our
482 earlier example, $days{'Feb'} can be written as $days{Feb} and the
483 quotes will be assumed automatically. But anything more complicated in
484 the subscript will be interpreted as an expression. This means for
485 example that "$version{2.0}++" is equivalent to "$version{2}++", not to
486 "$version{'2.0'}++".
487 There is a similar problem with interpolation with text that looks like
489 array or hash access notation. Placing a simple variable like $who
490 immediately in front of text like "[1]" or "{foo}" would cause the
491 variable to be interpolated as accessing an element of @who or a value
492 stored in %who:
493 $who = "Larry Wall";
495 print "$who[1] is the father of Perl.\n";
496 would attempt to access index 1 of an array named @who. Again, using
498 braces will prevent this from happening:
499 $who = "Larry Wall";
501 print "${who}[1] is the father of Perl.\n";
502 will be treated the same as
504 $who = "Larry Wall";
506 print $who . "[1] is the father of Perl.\n";
507 This notation also applies to more complex variable descriptions, such
509 as array or hash access with subscripts. For instance
510 @name = qw(Larry Curly Moe);
512 print "Also ${name[0]}[1] was a member\n";
513 Without the braces the above example would be parsed as a two level
515 array subscript in the @name array, and under "use strict" would likely
516 produce a fatal exception, as it would be parsed like this:
517 print "Also " . $name[0][1] . " was a member\n";
519 and not as the intended:
521 print "Also " . $name[0] . "[1] was a member\n";
523 A similar result may be derived by using a backslash on the first
525 character of the subscript or package notation that is not part of the
526 variable you want to access. Thus the above example could also be
527 written:
528 @name = qw(Larry Curly Moe);
530 print "Also $name[0]\[1] was a member\n";
531 however for some special variables (multi character caret variables)
533 the demarcated form using curly braces is the only way you can
534 reference the variable at all, and the only way you can access a
535 subscript of the variable via interpolation.
536 Consider the magic array "@{^CAPTURE}" which is populated by the regex
538 engine with the contents of all of the capture buffers in a pattern
539 (see perlvar and perlre). The only way you can access one of these
540 members inside of a string is via the braced (demarcated) form:
541 "abc"=~/(.)(.)(.)/
543 and print "Second buffer is ${^CAPTURE[1]}";
544 is equivalent to
546 "abc"=~/(.)(.)(.)/
548 and print "Second buffer is " . ${^CAPTURE}[1];
549 Saying "@^CAPTURE" is a syntax error, so it must be referenced as
551 "@{^CAPTURE}", and to access one of its elements in normal code you
552 would write " ${^CAPTURE}[1] ". However when interpolating in a string
553 "${^CAPTURE}[1]" would be equivalent to "${^CAPTURE} . "[1]"", which
554 does not even refer to the same variable! Thus the subscripts must also
555 be placed inside of the braces: "${^CAPTURE[1]}".
556 The demarcated form using curly braces can be used with all the
558 different types of variable access, including array and hash slices.
559 For instance code like the following:
560 @name = qw(Larry Curly Moe);
562 local $" = " and ";
563 print "My favorites were @{name[1,2]}.\n";
564 would output
566 My favorites were Curly and Moe.
568 Special floating point: infinity (Inf) and not-a-number (NaN)
570 Floating point values include the special values "Inf" and "NaN", for
572 infinity and not-a-number. The infinity can be also negative.
573 The infinity is the result of certain math operations that overflow the
575 floating point range, like 9**9**9. The not-a-number is the result
576 when the result is undefined or unrepresentable. Though note that you
577 cannot get "NaN" from some common "undefined" or "out-of-range"
578 operations like dividing by zero, or square root of a negative number,
579 since Perl generates fatal errors for those.
580 The infinity and not-a-number have their own special arithmetic rules.
582 The general rule is that they are "contagious": "Inf" plus one is
583 "Inf", and "NaN" plus one is "NaN". Where things get interesting is
584 when you combine infinities and not-a-numbers: "Inf" minus "Inf" and
585 "Inf" divided by "Inf" are "NaN" (while "Inf" plus "Inf" is "Inf" and
586 "Inf" times "Inf" is "Inf"). "NaN" is also curious in that it does not
587 equal any number, including itself: "NaN" != "NaN".
588 Perl doesn't understand "Inf" and "NaN" as numeric literals, but you
590 can have them as strings, and Perl will convert them as needed: "Inf" +
591 1. (You can, however, import them from the POSIX extension; "use POSIX
592 qw(Inf NaN);" and then use them as literals.)
593 Note that on input (string to number) Perl accepts "Inf" and "NaN" in
595 many forms. Case is ignored, and the Win32-specific forms like
596 "1.#INF" are understood, but on output the values are normalized to
597 "Inf" and "NaN".
598 Version Strings
600 A literal of the form "v1.20.300.4000" is parsed as a string composed
602 of characters with the specified ordinals. This form, known as
603 v-strings, provides an alternative, more readable way to construct
604 strings, rather than use the somewhat less readable interpolation form
605 "\x{1}\x{14}\x{12c}\x{fa0}". This is useful for representing Unicode
606 strings, and for comparing version "numbers" using the string
607 comparison operators, "cmp", "gt", "lt" etc. If there are two or more
608 dots in the literal, the leading "v" may be omitted.
609 print v9786; # prints SMILEY, "\x{263a}"
611 print v102.111.111; # prints "foo"
612 print 102.111.111; # same
613 Such literals are accepted by both "require" and "use" for doing a
615 version check. Note that using the v-strings for IPv4 addresses is not
616 portable unless you also use the inet_aton()/inet_ntoa() routines of
617 the Socket package.
618 Note that since Perl 5.8.1 the single-number v-strings (like "v65") are
620 not v-strings before the "=>" operator (which is usually used to
621 separate a hash key from a hash value); instead they are interpreted as
622 literal strings ('v65'). They were v-strings from Perl 5.6.0 to Perl
623 5.8.0, but that caused more confusion and breakage than good. Multi-
624 number v-strings like "v65.66" and 65.66.67 continue to be v-strings
625 always.
626 Special Literals
628 The special literals __FILE__, __LINE__, and __PACKAGE__ represent the
630 current filename, line number, and package name at that point in your
631 program. __SUB__ gives a reference to the current subroutine. They
632 may be used only as separate tokens; they will not be interpolated into
633 strings. If there is no current package (due to an empty "package;"
634 directive), __PACKAGE__ is the undefined value. (But the empty
635 "package;" is no longer supported, as of version 5.10.) Outside of a
636 subroutine, __SUB__ is the undefined value. __SUB__ is only available
637 in 5.16 or higher, and only with a "use v5.16" or "use feature
638 "current_sub"" declaration.
639 The two control characters ^D and ^Z, and the tokens __END__ and
641 __DATA__ may be used to indicate the logical end of the script before
642 the actual end of file. Any following text is ignored by the
643 interpreter unless read by the program as described below.
644 Text after __DATA__ may be read via the filehandle "PACKNAME::DATA",
646 where "PACKNAME" is the package that was current when the __DATA__
647 token was encountered. The filehandle is left open pointing to the
648 line after __DATA__. The program should "close DATA" when it is done
649 reading from it. (Leaving it open leaks filehandles if the module is
650 reloaded for any reason, so it's a safer practice to close it.) For
651 compatibility with older scripts written before __DATA__ was
652 introduced, __END__ behaves like __DATA__ in the top level script (but
653 not in files loaded with "require" or "do") and leaves the remaining
654 contents of the file accessible via "main::DATA".
655 while (my $line = <DATA>) { print $line; }
657 close DATA;
658 __DATA__
659 Hello world.
660 The "DATA" file handle by default has whatever PerlIO layers were in
662 place when Perl read the file to parse the source. Normally that means
663 that the file is being read bytewise, as if it were encoded in Latin-1,
664 but there are two major ways for it to be otherwise. Firstly, if the
665 "__END__"/"__DATA__" token is in the scope of a "use utf8" pragma then
666 the "DATA" handle will be in UTF-8 mode. And secondly, if the source
667 is being read from perl's standard input then the "DATA" file handle is
668 actually aliased to the "STDIN" file handle, and may be in UTF-8 mode
669 because of the "PERL_UNICODE" environment variable or perl's command-
670 line switches.
671 See SelfLoader for more description of __DATA__, and an example of its
673 use. Note that you cannot read from the DATA filehandle in a BEGIN
674 block: the BEGIN block is executed as soon as it is seen (during
675 compilation), at which point the corresponding __DATA__ (or __END__)
676 token has not yet been seen.
677 Barewords
679 A word that has no other interpretation in the grammar will be treated
681 as if it were a quoted string. These are known as "barewords". As
682 with filehandles and labels, a bareword that consists entirely of
683 lowercase letters risks conflict with future reserved words, and if you
684 use the "use warnings" pragma or the -w switch, Perl will warn you
685 about any such words. Perl limits barewords (like identifiers) to
686 about 250 characters. Future versions of Perl are likely to eliminate
687 these arbitrary limitations.
688 Some people may wish to outlaw barewords entirely. If you say
690 use strict 'subs';
692 then any bareword that would NOT be interpreted as a subroutine call
694 produces a compile-time error instead. The restriction lasts to the
695 end of the enclosing block. An inner block may countermand this by
696 saying "no strict 'subs'".
697 Array Interpolation
699 Arrays and slices are interpolated into double-quoted strings by
701 joining the elements with the delimiter specified in the $" variable
702 ($LIST_SEPARATOR if "use English;" is specified), space by default.
703 The following are equivalent:
704 $temp = join($", @ARGV);
706 system "echo $temp";
707 system "echo @ARGV";
709 Within search patterns (which also undergo double-quotish substitution)
711 there is an unfortunate ambiguity: Is "/$foo[bar]/" to be interpreted
712 as "/${foo}[bar]/" (where "[bar]" is a character class for the regular
713 expression) or as "/${foo[bar]}/" (where "[bar]" is the subscript to
714 array @foo)? If @foo doesn't otherwise exist, then it's obviously a
715 character class. If @foo exists, Perl takes a good guess about
716 "[bar]", and is almost always right. If it does guess wrong, or if
717 you're just plain paranoid, you can force the correct interpretation
718 with curly braces as above.
719 If you're looking for the information on how to use here-documents,
721 which used to be here, that's been moved to "Quote and Quote-like
722 Operators" in perlop.
723 List value constructors
725 List values are denoted by separating individual values by commas (and
726 enclosing the list in parentheses where precedence requires it):
727 (LIST)
729 In a context not requiring a list value, the value of what appears to
731 be a list literal is simply the value of the final element, as with the
732 C comma operator. For example,
733 @foo = ('cc', '-E', $bar);
735 assigns the entire list value to array @foo, but
737 $foo = ('cc', '-E', $bar);
739 assigns the value of variable $bar to the scalar variable $foo. Note
741 that the value of an actual array in scalar context is the length of
742 the array; the following assigns the value 3 to $foo:
743 @foo = ('cc', '-E', $bar);
745 $foo = @foo; # $foo gets 3
746 You may have an optional comma before the closing parenthesis of a list
748 literal, so that you can say:
749 @foo = (
751 1,
752 2,
753 3,
754 );
755 To use a here-document to assign an array, one line per element, you
757 might use an approach like this:
758 @sauces = <<End_Lines =~ m/(\S.*\S)/g;
760 normal tomato
761 spicy tomato
762 green chile
763 pesto
764 white wine
765 End_Lines
766 LISTs do automatic interpolation of sublists. That is, when a LIST is
768 evaluated, each element of the list is evaluated in list context, and
769 the resulting list value is interpolated into LIST just as if each
770 individual element were a member of LIST. Thus arrays and hashes lose
771 their identity in a LIST--the list
772 (@foo,@bar,&SomeSub,%glarch)
774 contains all the elements of @foo followed by all the elements of @bar,
776 followed by all the elements returned by the subroutine named SomeSub
777 called in list context, followed by the key/value pairs of %glarch. To
778 make a list reference that does NOT interpolate, see perlref.
779 The null list is represented by (). Interpolating it in a list has no
781 effect. Thus ((),(),()) is equivalent to (). Similarly, interpolating
782 an array with no elements is the same as if no array had been
783 interpolated at that point.
784 This interpolation combines with the facts that the opening and closing
786 parentheses are optional (except when necessary for precedence) and
787 lists may end with an optional comma to mean that multiple commas
788 within lists are legal syntax. The list "1,,3" is a concatenation of
789 two lists, "1," and 3, the first of which ends with that optional
790 comma. "1,,3" is "(1,),(3)" is "1,3" (And similarly for "1,,,3" is
791 "(1,),(,),3" is "1,3" and so on.) Not that we'd advise you to use this
792 obfuscation.
793 A list value may also be subscripted like a normal array. You must put
795 the list in parentheses to avoid ambiguity. For example:
796 # Stat returns list value.
798 $time = (stat($file))[8];
799 # SYNTAX ERROR HERE.
801 $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
802 # Find a hex digit.
804 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
805 # A "reverse comma operator".
807 return (pop(@foo),pop(@foo))[0];
808 Lists may be assigned to only when each element of the list is itself
810 legal to assign to:
811 ($x, $y, $z) = (1, 2, 3);
813 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
815 An exception to this is that you may assign to "undef" in a list. This
817 is useful for throwing away some of the return values of a function:
818 ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
820 As of Perl 5.22, you can also use "(undef)x2" instead of "undef,
822 undef". (You can also do "($x) x 2", which is less useful, because it
823 assigns to the same variable twice, clobbering the first value
824 assigned.)
825 When you assign a list of scalars to an array, all previous values in
827 that array are wiped out and the number of elements in the array will
828 now be equal to the number of elements in the right-hand list -- the
829 list from which assignment was made. The array will automatically
830 resize itself to precisely accommodate each element in the right-hand
831 list.
832 use warnings;
834 my (@xyz, $x, $y, $z);
835 @xyz = (1, 2, 3);
837 print "@xyz\n"; # 1 2 3
838 @xyz = ('al', 'be', 'ga', 'de');
840 print "@xyz\n"; # al be ga de
841 @xyz = (101, 102);
843 print "@xyz\n"; # 101 102
844 When, however, you assign a list of scalars to another list of scalars,
846 the results differ according to whether the left-hand list -- the list
847 being assigned to -- has the same, more or fewer elements than the
848 right-hand list.
849 ($x, $y, $z) = (1, 2, 3);
851 print "$x $y $z\n"; # 1 2 3
852 ($x, $y, $z) = ('al', 'be', 'ga', 'de');
854 print "$x $y $z\n"; # al be ga
855 ($x, $y, $z) = (101, 102);
857 print "$x $y $z\n"; # 101 102
858 # Use of uninitialized value $z in concatenation (.)
859 # or string at [program] line [line number].
860 If the number of scalars in the left-hand list is less than that in the
862 right-hand list, the "extra" scalars in the right-hand list will simply
863 not be assigned.
864 If the number of scalars in the left-hand list is greater than that in
866 the left-hand list, the "missing" scalars will become undefined.
867 ($x, $y, $z) = (101, 102);
869 for my $el ($x, $y, $z) {
870 (defined $el) ? print "$el " : print "<undef>";
871 }
872 print "\n";
873 # 101 102 <undef>
874 List assignment in scalar context returns the number of elements
876 produced by the expression on the right side of the assignment:
877 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
879 $x = (($foo,$bar) = f()); # set $x to f()'s return count
880 This is handy when you want to do a list assignment in a Boolean
882 context, because most list functions return a null list when finished,
883 which when assigned produces a 0, which is interpreted as FALSE.
884 It's also the source of a useful idiom for executing a function or
886 performing an operation in list context and then counting the number of
887 return values, by assigning to an empty list and then using that
888 assignment in scalar context. For example, this code:
889 $count = () = $string =~ /\d+/g;
891 will place into $count the number of digit groups found in $string.
893 This happens because the pattern match is in list context (since it is
894 being assigned to the empty list), and will therefore return a list of
895 all matching parts of the string. The list assignment in scalar
896 context will translate that into the number of elements (here, the
897 number of times the pattern matched) and assign that to $count. Note
898 that simply using
899 $count = $string =~ /\d+/g;
901 would not have worked, since a pattern match in scalar context will
903 only return true or false, rather than a count of matches.
904 The final element of a list assignment may be an array or a hash:
906 ($x, $y, @rest) = split;
908 my($x, $y, %rest) = @_;
909 You can actually put an array or hash anywhere in the list, but the
911 first one in the list will soak up all the values, and anything after
912 it will become undefined. This may be useful in a my() or local().
913 A hash can be initialized using a literal list holding pairs of items
915 to be interpreted as a key and a value:
916 # same as map assignment above
918 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
919 While literal lists and named arrays are often interchangeable, that's
921 not the case for hashes. Just because you can subscript a list value
922 like a normal array does not mean that you can subscript a list value
923 as a hash. Likewise, hashes included as parts of other lists
924 (including parameters lists and return lists from functions) always
925 flatten out into key/value pairs. That's why it's good to use
926 references sometimes.
927 It is often more readable to use the "=>" operator between key/value
929 pairs. The "=>" operator is mostly just a more visually distinctive
930 synonym for a comma, but it also arranges for its left-hand operand to
931 be interpreted as a string if it's a bareword that would be a legal
932 simple identifier. "=>" doesn't quote compound identifiers, that
933 contain double colons. This makes it nice for initializing hashes:
934 %map = (
936 red => 0x00f,
937 blue => 0x0f0,
938 green => 0xf00,
939 );
940 or for initializing hash references to be used as records:
942 $rec = {
944 witch => 'Mable the Merciless',
945 cat => 'Fluffy the Ferocious',
946 date => '10/31/1776',
947 };
948 or for using call-by-named-parameter to complicated functions:
950 $field = $query->radio_group(
952 name => 'group_name',
953 values => ['eenie','meenie','minie'],
954 default => 'meenie',
955 linebreak => 'true',
956 labels => \%labels
957 );
958 Note that just because a hash is initialized in that order doesn't mean
960 that it comes out in that order. See "sort" in perlfunc for examples
961 of how to arrange for an output ordering.
962 If a key appears more than once in the initializer list of a hash, the
964 last occurrence wins:
965 %circle = (
967 center => [5, 10],
968 center => [27, 9],
969 radius => 100,
970 color => [0xDF, 0xFF, 0x00],
971 radius => 54,
972 );
973 # same as
975 %circle = (
976 center => [27, 9],
977 color => [0xDF, 0xFF, 0x00],
978 radius => 54,
979 );
980 This can be used to provide overridable configuration defaults:
982 # values in %args take priority over %config_defaults
984 %config = (%config_defaults, %args);
985 Subscripts
987 An array can be accessed one scalar at a time by specifying a dollar
988 sign ("$"), then the name of the array (without the leading "@"), then
989 the subscript inside square brackets. For example:
990 @myarray = (5, 50, 500, 5000);
992 print "The Third Element is", $myarray[2], "\n";
993 The array indices start with 0. A negative subscript retrieves its
995 value from the end. In our example, $myarray[-1] would have been 5000,
996 and $myarray[-2] would have been 500.
997 Hash subscripts are similar, only instead of square brackets curly
999 brackets are used. For example:
1000 %scientists =
1002 (
1003 "Newton" => "Isaac",
1004 "Einstein" => "Albert",
1005 "Darwin" => "Charles",
1006 "Feynman" => "Richard",
1007 );
1008 print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
1010 You can also subscript a list to get a single element from it:
1012 $dir = (getpwnam("daemon"))[7];
1014 Multi-dimensional array emulation
1016 Multidimensional arrays may be emulated by subscripting a hash with a
1017 list. The elements of the list are joined with the subscript separator
1018 (see "$;" in perlvar).
1019 $foo{$x,$y,$z}
1021 is equivalent to
1023 $foo{join($;, $x, $y, $z)}
1025 The default subscript separator is "\034", the same as SUBSEP in awk.
1027 Slices
1029 A slice accesses several elements of a list, an array, or a hash
1030 simultaneously using a list of subscripts. It's more convenient than
1031 writing out the individual elements as a list of separate scalar
1032 values.
1033 ($him, $her) = @folks[0,-1]; # array slice
1035 @them = @folks[0 .. 3]; # array slice
1036 ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
1037 ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
1038 Since you can assign to a list of variables, you can also assign to an
1040 array or hash slice.
1041 @days[3..5] = qw/Wed Thu Fri/;
1043 @colors{'red','blue','green'}
1044 = (0xff0000, 0x0000ff, 0x00ff00);
1045 @folks[0, -1] = @folks[-1, 0];
1046 The previous assignments are exactly equivalent to
1048 ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
1050 ($colors{'red'}, $colors{'blue'}, $colors{'green'})
1051 = (0xff0000, 0x0000ff, 0x00ff00);
1052 ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
1053 Since changing a slice changes the original array or hash that it's
1055 slicing, a "foreach" construct will alter some--or even all--of the
1056 values of the array or hash.
1057 foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
1059 foreach (@hash{qw[key1 key2]}) {
1061 s/^\s+//; # trim leading whitespace
1062 s/\s+$//; # trim trailing whitespace
1063 s/\b(\w)(\w*)\b/\u$1\L$2/g; # "titlecase" words
1064 }
1065 As a special exception, when you slice a list (but not an array or a
1067 hash), if the list evaluates to empty, then taking a slice of that
1068 empty list will always yield the empty list in turn. Thus:
1069 @a = ()[0,1]; # @a has no elements
1071 @b = (@a)[0,1]; # @b has no elements
1072 @c = (sub{}->())[0,1]; # @c has no elements
1073 @d = ('a','b')[0,1]; # @d has two elements
1074 @e = (@d)[0,1,8,9]; # @e has four elements
1075 @f = (@d)[8,9]; # @f has two elements
1076 This makes it easy to write loops that terminate when a null list is
1078 returned:
1079 while ( ($home, $user) = (getpwent)[7,0] ) {
1081 printf "%-8s %s\n", $user, $home;
1082 }
1083 As noted earlier in this document, the scalar sense of list assignment
1085 is the number of elements on the right-hand side of the assignment.
1086 The null list contains no elements, so when the password file is
1087 exhausted, the result is 0, not 2.
1088 Slices in scalar context return the last item of the slice.
1090 @a = qw/first second third/;
1092 %h = (first => 'A', second => 'B');
1093 $t = @a[0, 1]; # $t is now 'second'
1094 $u = @h{'first', 'second'}; # $u is now 'B'
1095 If you're confused about why you use an '@' there on a hash slice
1097 instead of a '%', think of it like this. The type of bracket (square
1098 or curly) governs whether it's an array or a hash being looked at. On
1099 the other hand, the leading symbol ('$' or '@') on the array or hash
1100 indicates whether you are getting back a singular value (a scalar) or a
1101 plural one (a list).
1102 Key/Value Hash Slices
1104 Starting in Perl 5.20, a hash slice operation with the % symbol is a
1106 variant of slice operation returning a list of key/value pairs rather
1107 than just values:
1108 %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
1110 %subset = %h{'foo', 'bar'}; # key/value hash slice
1111 # %subset is now (foo => 3, bar => 8)
1112 %removed = delete %h{'foo', 'bar'};
1113 # %removed is now (foo => 3, bar => 8)
1114 # %h is now (blonk => 2, squink => 5)
1115 However, the result of such a slice cannot be localized or assigned to.
1117 These are otherwise very much consistent with hash slices using the @
1118 symbol.
1119 Index/Value Array Slices
1121 Similar to key/value hash slices (and also introduced in Perl 5.20),
1123 the % array slice syntax returns a list of index/value pairs:
1124 @a = "a".."z";
1126 @list = %a[3,4,6];
1127 # @list is now (3, "d", 4, "e", 6, "g")
1128 @removed = delete %a[3,4,6]
1129 # @removed is now (3, "d", 4, "e", 6, "g")
1130 # @list[3,4,6] are now undef
1131 Note that calling "delete" on array values is strongly discouraged.
1133 Typeglobs and Filehandles
1135 Perl uses an internal type called a typeglob to hold an entire symbol
1136 table entry. The type prefix of a typeglob is a "*", because it
1137 represents all types. This used to be the preferred way to pass arrays
1138 and hashes by reference into a function, but now that we have real
1139 references, this is seldom needed.
1140 The main use of typeglobs in modern Perl is create symbol table
1142 aliases. This assignment:
1143 *this = *that;
1145 makes $this an alias for $that, @this an alias for @that, %this an
1147 alias for %that, &this an alias for &that, etc. Much safer is to use a
1148 reference. This:
1149 local *Here::blue = \$There::green;
1151 temporarily makes $Here::blue an alias for $There::green, but doesn't
1153 make @Here::blue an alias for @There::green, or %Here::blue an alias
1154 for %There::green, etc. See "Symbol Tables" in perlmod for more
1155 examples of this. Strange though this may seem, this is the basis for
1156 the whole module import/export system.
1157 Another use for typeglobs is to pass filehandles into a function or to
1159 create new filehandles. If you need to use a typeglob to save away a
1160 filehandle, do it this way:
1161 $fh = *STDOUT;
1163 or perhaps as a real reference, like this:
1165 $fh = \*STDOUT;
1167 See perlsub for examples of using these as indirect filehandles in
1169 functions.
1170 Typeglobs are also a way to create a local filehandle using the local()
1172 operator. These last until their block is exited, but may be passed
1173 back. For example:
1174 sub newopen {
1176 my $path = shift;
1177 local *FH; # not my!
1178 open (FH, $path) or return undef;
1179 return *FH;
1180 }
1181 $fh = newopen('/etc/passwd');
1182 Now that we have the *foo{THING} notation, typeglobs aren't used as
1184 much for filehandle manipulations, although they're still needed to
1185 pass brand new file and directory handles into or out of functions.
1186 That's because *HANDLE{IO} only works if HANDLE has already been used
1187 as a handle. In other words, *FH must be used to create new symbol
1188 table entries; *foo{THING} cannot. When in doubt, use *FH.
1189 All functions that are capable of creating filehandles (open(),
1191 opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
1192 automatically create an anonymous filehandle if the handle passed to
1193 them is an uninitialized scalar variable. This allows the constructs
1194 such as "open(my $fh, ...)" and "open(local $fh,...)" to be used to
1195 create filehandles that will conveniently be closed automatically when
1196 the scope ends, provided there are no other references to them. This
1197 largely eliminates the need for typeglobs when opening filehandles that
1198 must be passed around, as in the following example:
1199 sub myopen {
1201 open my $fh, "@_"
1202 or die "Can't open '@_': $!";
1203 return $fh;
1204 }
1205 {
1207 my $f = myopen("</etc/motd");
1208 print <$f>;
1209 # $f implicitly closed here
1210 }
1211 Note that if an initialized scalar variable is used instead the result
1213 is different: "my $fh='zzz'; open($fh, ...)" is equivalent to "open(
1214 *{'zzz'}, ...)". "use strict 'refs'" forbids such practice.
1215 Another way to create anonymous filehandles is with the Symbol module
1217 or with the IO::Handle module and its ilk. These modules have the
1218 advantage of not hiding different types of the same name during the
1219 local(). See the bottom of "open" in perlfunc for an example.
1220
1222 See perlvar for a description of Perl's built-in variables and a
1223 discussion of legal variable names. See perlref, perlsub, and "Symbol
1224 Tables" in perlmod for more discussion on typeglobs and the *foo{THING}
1225 syntax.
1226perl v5.38.2 2023-11-30 PERLDATA(1)