1PERLTRAP(1) Perl Programmers Reference Guide PERLTRAP(1)
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6 perltrap - Perl traps for the unwary
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9 The biggest trap of all is forgetting to "use warnings" or use the -w
10 switch; see warnings and "-w" in perlrun. The second biggest trap is
11 not making your entire program runnable under "use strict". The third
12 biggest trap is not reading the list of changes in this version of
13 Perl; see perldelta.
14 Awk Traps
16 Accustomed awk users should take special note of the following:
17 • A Perl program executes only once, not once for each input line.
19 You can do an implicit loop with "-n" or "-p".
20 • The English module, loaded via
22 use English;
24 allows you to refer to special variables (like $/) with names (like
26 $RS), as though they were in awk; see perlvar for details.
27 • Semicolons are required after all simple statements in Perl (except
29 at the end of a block). Newline is not a statement delimiter.
30 • Curly brackets are required on "if"s and "while"s.
32 • Variables begin with "$", "@" or "%" in Perl.
34 • Arrays index from 0. Likewise string positions in substr() and
36 index().
37 • You have to decide whether your array has numeric or string
39 indices.
40 • Hash values do not spring into existence upon mere reference.
42 • You have to decide whether you want to use string or numeric
44 comparisons.
45 • Reading an input line does not split it for you. You get to split
47 it to an array yourself. And the split() operator has different
48 arguments than awk's.
49 • The current input line is normally in $_, not $0. It generally
51 does not have the newline stripped. ($0 is the name of the program
52 executed.) See perlvar.
53 • $<digit> does not refer to fields--it refers to substrings matched
55 by the last match pattern.
56 • The print() statement does not add field and record separators
58 unless you set $, and "$\". You can set $OFS and $ORS if you're
59 using the English module.
60 • You must open your files before you print to them.
62 • The range operator is "..", not comma. The comma operator works as
64 in C.
65 • The match operator is "=~", not "~". ("~" is the one's complement
67 operator, as in C.)
68 • The exponentiation operator is "**", not "^". "^" is the XOR
70 operator, as in C. (You know, one could get the feeling that awk
71 is basically incompatible with C.)
72 • The concatenation operator is ".", not the null string. (Using the
74 null string would render "/pat/ /pat/" unparsable, because the
75 third slash would be interpreted as a division operator--the
76 tokenizer is in fact slightly context sensitive for operators like
77 "/", "?", and ">". And in fact, "." itself can be the beginning of
78 a number.)
79 • The "next", "exit", and "continue" keywords work differently.
81 • The following variables work differently:
83 Awk Perl
85 ARGC scalar @ARGV (compare with $#ARGV)
86 ARGV[0] $0
87 FILENAME $ARGV
88 FNR $. - something
89 FS (whatever you like)
90 NF $#Fld, or some such
91 NR $.
92 OFMT $#
93 OFS $,
94 ORS $\
95 RLENGTH length($&)
96 RS $/
97 RSTART length($`)
98 SUBSEP $;
99 • You cannot set $RS to a pattern, only a string.
101 • When in doubt, run the awk construct through a2p and see what it
103 gives you.
104 C/C++ Traps
106 Cerebral C and C++ programmers should take note of the following:
107 • Curly brackets are required on "if"'s and "while"'s.
109 • You must use "elsif" rather than "else if".
111 • The "break" and "continue" keywords from C become in Perl "last"
113 and "next", respectively. Unlike in C, these do not work within a
114 "do { } while" construct. See "Loop Control" in perlsyn.
115 • The switch statement is called "given"/"when" and only available in
117 perl 5.10 or newer. See "Switch Statements" in perlsyn.
118 • Variables begin with "$", "@" or "%" in Perl.
120 • Comments begin with "#", not "/*" or "//". Perl may interpret
122 C/C++ comments as division operators, unterminated regular
123 expressions or the defined-or operator.
124 • You can't take the address of anything, although a similar operator
126 in Perl is the backslash, which creates a reference.
127 • "ARGV" must be capitalized. $ARGV[0] is C's "argv[1]", and
129 "argv[0]" ends up in $0.
130 • System calls such as link(), unlink(), rename(), etc. return
132 nonzero for success, not 0. (system(), however, returns zero for
133 success.)
134 • Signal handlers deal with signal names, not numbers. Use "kill -l"
136 to find their names on your system.
137 JavaScript Traps
139 Judicious JavaScript programmers should take note of the following:
140 • In Perl, binary "+" is always addition. "$string1 + $string2"
142 converts both strings to numbers and then adds them. To
143 concatenate two strings, use the "." operator.
144 • The "+" unary operator doesn't do anything in Perl. It exists to
146 avoid syntactic ambiguities.
147 • Unlike "for...in", Perl's "for" (also spelled "foreach") does not
149 allow the left-hand side to be an arbitrary expression. It must be
150 a variable:
151 for my $variable (keys %hash) {
153 ...
154 }
155 Furthermore, don't forget the "keys" in there, as "foreach my $kv
157 (%hash) {}" iterates over the keys and values, and is generally not
158 useful ($kv would be a key, then a value, and so on).
159 • To iterate over the indices of an array, use "foreach my $i (0 ..
161 $#array) {}". "foreach my $v (@array) {}" iterates over the
162 values.
163 • Perl requires braces following "if", "while", "foreach", etc.
165 • In Perl, "else if" is spelled "elsif".
167 • "? :" has higher precedence than assignment. In JavaScript, one
169 can write:
170 condition ? do_something() : variable = 3
172 and the variable is only assigned if the condition is false. In
174 Perl, you need parentheses:
175 $condition ? do_something() : ($variable = 3);
177 Or just use "if".
179 • Perl requires semicolons to separate statements.
181 • Variables declared with "my" only affect code after the
183 declaration. You cannot write "$x = 1; my $x;" and expect the
184 first assignment to affect the same variable. It will instead
185 assign to an $x declared previously in an outer scope, or to a
186 global variable.
187 Note also that the variable is not visible until the following
189 statement. This means that in "my $x = 1 + $x" the second $x
190 refers to one declared previously.
191 • "my" variables are scoped to the current block, not to the current
193 function. If you write "{my $x;} $x;", the second $x does not
194 refer to the one declared inside the block.
195 • An object's members cannot be made accessible as variables. The
197 closest Perl equivalent to "with(object) { method() }" is "for",
198 which can alias $_ to the object:
199 for ($object) {
201 $_->method;
202 }
203 • The object or class on which a method is called is passed as one of
205 the method's arguments, not as a separate "this" value.
206 Sed Traps
208 Seasoned sed programmers should take note of the following:
209 • A Perl program executes only once, not once for each input line.
211 You can do an implicit loop with "-n" or "-p".
212 • Backreferences in substitutions use "$" rather than "\".
214 • The pattern matching metacharacters "(", ")", and "|" do not have
216 backslashes in front.
217 • The range operator is "...", rather than comma.
219 Shell Traps
221 Sharp shell programmers should take note of the following:
222 • The backtick operator does variable interpolation without regard to
224 the presence of single quotes in the command.
225 • The backtick operator does no translation of the return value,
227 unlike csh.
228 • Shells (especially csh) do several levels of substitution on each
230 command line. Perl does substitution in only certain constructs
231 such as double quotes, backticks, angle brackets, and search
232 patterns.
233 • Shells interpret scripts a little bit at a time. Perl compiles the
235 entire program before executing it (except for "BEGIN" blocks,
236 which execute at compile time).
237 • The arguments are available via @ARGV, not $1, $2, etc.
239 • The environment is not automatically made available as separate
241 scalar variables.
242 • The shell's "test" uses "=", "!=", "<" etc for string comparisons
244 and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the
245 reverse of Perl, which uses "eq", "ne", "lt" for string
246 comparisons, and "==", "!=" "<" etc for numeric comparisons.
247 Perl Traps
249 Practicing Perl Programmers should take note of the following:
250 • Remember that many operations behave differently in a list context
252 than they do in a scalar one. See perldata for details.
253 • Avoid barewords if you can, especially all lowercase ones. You
255 can't tell by just looking at it whether a bareword is a function
256 or a string. By using quotes on strings and parentheses on
257 function calls, you won't ever get them confused.
258 • You cannot discern from mere inspection which builtins are unary
260 operators (like chop() and chdir()) and which are list operators
261 (like print() and unlink()). (Unless prototyped, user-defined
262 subroutines can only be list operators, never unary ones.) See
263 perlop and perlsub.
264 • People have a hard time remembering that some functions default to
266 $_, or @ARGV, or whatever, but that others which you might expect
267 to do not.
268 • The <FH> construct is not the name of the filehandle, it is a
270 readline operation on that handle. The data read is assigned to $_
271 only if the file read is the sole condition in a while loop:
272 while (<FH>) { }
274 while (defined($_ = <FH>)) { }..
275 <FH>; # data discarded!
276 • Remember not to use "=" when you need "=~"; these two constructs
278 are quite different:
279 $x = /foo/;
281 $x =~ /foo/;
282 • The "do {}" construct isn't a real loop that you can use loop
284 control on.
285 • Use my() for local variables whenever you can get away with it (but
287 see perlform for where you can't). Using local() actually gives a
288 local value to a global variable, which leaves you open to
289 unforeseen side-effects of dynamic scoping.
290 • If you localize an exported variable in a module, its exported
292 value will not change. The local name becomes an alias to a new
293 value but the external name is still an alias for the original.
294 As always, if any of these are ever officially declared as bugs,
296 they'll be fixed and removed.
297perl v5.38.2 2023-11-30 PERLTRAP(1)