=head1 DESCRIPTION
This page describes the syntax of regular expressions in Perl. For a
-description of how to actually I<use> regular expressions in matching
+description of how to I<use> regular expressions in matching
operations, plus various examples of the same, see C<m//> and C<s///> in
L<perlop>.
-The matching operations can
-have various modifiers, some of which relate to the interpretation of
-the regular expression inside. These are:
+The matching operations can have various modifiers. The modifiers
+which relate to the interpretation of the regular expression inside
+are listed below. For the modifiers that alter the behaviour of the
+operation, see L<perlop/"m//"> and L<perlop/"s//">.
- i Do case-insensitive pattern matching.
- m Treat string as multiple lines.
- s Treat string as single line.
- x Extend your pattern's legibility with whitespace and comments.
+=over 4
+
+=item i
+
+Do case-insensitive pattern matching.
+
+If C<use locale> is in effect, the case map is taken from the current
+locale. See L<perllocale>.
+
+=item m
+
+Treat string as multiple lines. That is, change "^" and "$" from matching
+at only the very start or end of the string to the start or end of any
+line anywhere within the string,
+
+=item s
+
+Treat string as single line. That is, change "." to match any character
+whatsoever, even a newline, which it normally would not match.
+
+=item x
+
+Extend your pattern's legibility by permitting whitespace and comments.
+
+=back
These are usually written as "the C</x> modifier", even though the delimiter
in question might not actually be a slash. In fact, any of these
the new C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
-the regular expression parser to ignore whitespace that is not
-backslashed or within a character class. You can use this to break up
+the regular expression parser to ignore whitespace that is neither
+backslashed nor within a character class. You can use this to break up
your regular expression into (slightly) more readable parts. The C<#>
character is also treated as a metacharacter introducing a comment,
-just as in ordinary Perl code. Taken together, these features go a
-long way towards making Perl 5 a readable language. See the C comment
-deletion code in L<perlop>.
+just as in ordinary Perl code. This also means that if you want real
+whitespace or C<#> characters in the pattern that you'll have to either
+escape them or encode them using octal or hex escapes. Taken together,
+these features go a long way towards making Perl's regular expressions
+more readable. See the C comment deletion code in L<perlop>.
=head2 Regular Expressions
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match only at the
-beginning of the string, the "$" character only at the end (or before the
+By default, the "^" character is guaranteed to match at only the
+beginning of the string, the "$" character at only the end (or before the
newline at the end) and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines
will not be matched by "^" or "$". You may, however, wish to treat a
newline within the string, and "$" will match before any newline. At the
cost of a little more overhead, you can do this by using the /m modifier
on the pattern match operator. (Older programs did this by setting C<$*>,
-but this practice is deprecated in Perl 5.)
+but this practice is now deprecated.)
To facilitate multi-line substitutions, the "." character never matches a
-newline unless you use the C</s> modifier, which tells Perl to pretend
+newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't. The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
code that sets it in another module.
to integral values less than 65536.
By default, a quantified subpattern is "greedy", that is, it will match as
-many times as possible without causing the rest pattern not to match. The
-standard quantifiers are all "greedy", in that they match as many
-occurrences as possible (given a particular starting location) without
-causing the pattern to fail. If you want it to match the minimum number
-of times possible, follow the quantifier with a "?" after any of them.
-Note that the meanings don't change, just the "gravity":
+many times as possible (given a particular starting location) while still
+allowing the rest of the pattern to match. If you want it to match the
+minimum number of times possible, follow the quantifier with a "?". Note
+that the meanings don't change, just the "greediness":
*? Match 0 or more times
+? Match 1 or more times
{n,}? Match at least n times
{n,m}? Match at least n but not more than m times
-Since patterns are processed as double quoted strings, the following
+Because patterns are processed as double quoted strings, the following
also work:
- \t tab
- \n newline
- \r return
- \f form feed
- \a alarm (bell)
- \e escape (think troff)
+ \t tab (HT, TAB)
+ \n newline (LF, NL)
+ \r return (CR)
+ \f form feed (FF)
+ \a alarm (bell) (BEL)
+ \e escape (think troff) (ESC)
\033 octal char (think of a PDP-11)
\x1B hex char
\c[ control char
\E end case modification (think vi)
\Q quote regexp metacharacters till \E
+If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
+and <\U> is taken from the current locale. See L<perllocale>.
+
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
\D Match a non-digit character
Note that C<\w> matches a single alphanumeric character, not a whole
-word. To match a word you'd need to say C<\w+>. You may use C<\w>,
-C<\W>, C<\s>, C<\S>, C<\d> and C<\D> within character classes (though not
-as either end of a range).
+word. To match a word you'd need to say C<\w+>. If C<use locale> is in
+effect, the list of alphabetic characters generated by C<\w> is taken
+from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
+C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
+either end of a range).
Perl defines the following zero-width assertions:
\b Match a word boundary
\B Match a non-(word boundary)
- \A Match only at beginning of string
- \Z Match only at end of string (or before newline at the end)
- \G Match only where previous m//g left off
+ \A Match at only beginning of string
+ \Z Match at only end of string (or before newline at the end)
+ \G Match only where previous m//g left off (works only with /g)
A word boundary (C<\b>) is defined as a spot between two characters that
-has a C<\w> on one side of it and and a C<\W> on the other side of it (in
+has a C<\w> on one side of it and a C<\W> on the other side of it (in
either order), counting the imaginary characters off the beginning and
end of the string as matching a C<\W>. (Within character classes C<\b>
represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
just like "^" and "$" except that they won't match multiple times when the
C</m> modifier is used, while "^" and "$" will match at every internal line
boundary. To match the actual end of the string, not ignoring newline,
-you can use C<\Z(?!\n)>.
+you can use C<\Z(?!\n)>. The C<\G> assertion can be used to chain global
+matches (using C<m//g>), as described in
+L<perlop/"Regexp Quote-Like Operators">.
-When the bracketing construct C<( ... )> is used, \<digit> matches the
+It is also useful when writing C<lex>-like scanners, when you have several
+regexps which you want to match against consequent substrings of your
+string, see the previous reference.
+The actual location where C<\G> will match can also be influenced
+by using C<pos()> as an lvalue. See L<perlfunc/pos>.
+
+When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
digit'th substring. Outside of the pattern, always use "$" instead of "\"
-in front of the digit. (The \<digit> notation can on rare occasion work
+in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
outside the current pattern, this should not be relied upon. See the
-WARNING below.) The scope of $<digit> (and C<$`>, C<$&>, and C<$')>
+WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
extends to the end of the enclosing BLOCK or eval string, or to the next
successful pattern match, whichever comes first. If you want to use
-parentheses to delimit subpattern (e.g. a set of alternatives) without
-saving it as a subpattern, follow the ( with a ?.
+parentheses to delimit a subpattern (e.g., a set of alternatives) without
+saving it as a subpattern, follow the ( with a ?:.
You may have as many parentheses as you wish. If you have more
than 9 substrings, the variables $10, $11, ... refer to the
corresponding substring. Within the pattern, \10, \11, etc. refer back
-to substrings if there have been at least that many left parens before
+to substrings if there have been at least that many left parentheses before
the backreference. Otherwise (for backward compatibility) \10 is the
same as \010, a backspace, and \11 the same as \011, a tab. And so
on. (\1 through \9 are always backreferences.)
C<$+> returns whatever the last bracket match matched. C<$&> returns the
-entire matched string. ($0 used to return the same thing, but not any
+entire matched string. (C<$0> used to return the same thing, but not any
more.) C<$`> returns everything before the matched string. C<$'> returns
everything after the matched string. Examples:
$seconds = $3;
}
+Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
+the program, it has to provide them on each and every pattern match.
+This can slow your program down. The same mechanism that handles
+these provides for the use of $1, $2, etc., so you pay the same price
+for each regexp that contains capturing parentheses. But if you never
+use $&, etc., in your script, then regexps I<without> capturing
+parentheses won't be penalized. So avoid $&, $', and $` if you can,
+but if you can't (and some algorithms really appreciate them), once
+you've used them once, use them at will, because you've already paid
+the price.
+
You will note that all backslashed metacharacters in Perl are
alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression
languages, there are no backslashed symbols that aren't alphanumeric.
-So anything that looks like \\, \(, \), \<, \>, \{, or \} is always
+So anything that looks like \\, \(, \), \E<lt>, \E<gt>, \{, or \} is always
interpreted as a literal character, not a metacharacter. This makes it
simple to quote a string that you want to use for a pattern but that
-you are afraid might contain metacharacters. Simply quote all the
+you are afraid might contain metacharacters. Quote simply all the
non-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
-You can also use the built-in quotemeta() function to do this.
+You can also use the builtin quotemeta() function to do this.
An even easier way to quote metacharacters right in the match operator
is to say
/$unquoted\Q$quoted\E$unquoted/
-Perl 5 defines a consistent extension syntax for regular expressions.
-The syntax is a pair of parens with a question mark as the first thing
-within the parens (this was a syntax error in Perl 4). The character
-after the question mark gives the function of the extension. Several
-extensions are already supported:
+Perl defines a consistent extension syntax for regular expressions.
+The syntax is a pair of parentheses with a question mark as the first
+thing within the parentheses (this was a syntax error in older
+versions of Perl). The character after the question mark gives the
+function of the extension. Several extensions are already supported:
=over 10
=item (?:regexp)
-This groups things like "()" but doesn't make backrefences like "()" does. So
+This groups things like "()" but doesn't make backreferences like "()" does. So
split(/\b(?:a|b|c)\b/)
"bar" that is preceded by something which is not "foo". That's because
the C<(?!foo)> is just saying that the next thing cannot be "foo"--and
it's not, it's a "bar", so "foobar" will match. You would have to do
-something like C</(?foo)...bar/> for that. We say "like" because there's
+something like C</(?!foo)...bar/> for that. We say "like" because there's
the case of your "bar" not having three characters before it. You could
cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
easier just to say:
One or more embedded pattern-match modifiers. This is particularly
useful for patterns that are specified in a table somewhere, some of
which want to be case sensitive, and some of which don't. The case
-insensitive ones merely need to include C<(?i)> at the front of the
+insensitive ones need to include merely C<(?i)> at the front of the
pattern. For example:
$pattern = "foobar";
finds a possible match right at the beginning of the string, and loads up
$1 with "Foo". However, as soon as the matching engine sees that there's
no whitespace following the "Foo" that it had saved in $1, it realizes its
-mistake and starts over again one character after where it had had the
+mistake and starts over again one character after where it had the
tentative match. This time it goes all the way until the next occurrence
of "foo". The complete regular expression matches this time, and you get
the expected output of "table follows foo."
whole string. As C<\d*> can match on an empty string the complete
regular expression matched successfully.
- Beginning is <I have 2: 53147>, number is <>.
+ Beginning is <I have 2 numbers: 53147>, number is <>.
Here are some variants, most of which don't work:
regular expression is merely a set of assertions that gives a definition
of success. There may be 0, 1, or several different ways that the
definition might succeed against a particular string. And if there are
-multiple ways it might succeed, you need to understand backtracking in
-order to know which variety of success you will achieve.
+multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve.
When using lookahead assertions and negations, this can all get even
-tricker. Imagine you'd like to find a sequence of nondigits not
+tricker. Imagine you'd like to find a sequence of non-digits not
followed by "123". You might try to write that as
$_ = "ABC123";
3: got AB
4: got ABC
-You might have expected test 3 to fail because it just seems to a more
+You might have expected test 3 to fail because it seems to a more
general purpose version of test 1. The important difference between
them is that test 3 contains a quantifier (C<\D*>) and so can use
backtracking, whereas test 1 will not. What's happening is
that you've asked "Is it true that at the start of $x, following 0 or more
-nondigits, you have something that's not 123?" If the pattern matcher had
+non-digits, you have something that's not 123?" If the pattern matcher had
let C<\D*> expand to "ABC", this would have caused the whole pattern to
-fail.
+fail.
The search engine will initially match C<\D*> with "ABC". Then it will
try to match C<(?!123> with "123" which, of course, fails. But because
a quantifier (C<\D*>) has been used in the regular expression, the
search engine can backtrack and retry the match differently
-in the hope of matching the complete regular expression.
+in the hope of matching the complete regular expression.
-Well now,
+Well now,
the pattern really, I<really> wants to succeed, so it uses the
-standard regexp backoff-and-retry and lets C<\D*> expand to just "AB" this
+standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
time. Now there's indeed something following "AB" that is not
"123". It's in fact "C123", which suffices.
Any single character matches itself, unless it is a I<metacharacter>
with a special meaning described here or above. You can cause
characters which normally function as metacharacters to be interpreted
-literally by prefixing them with a "\" (e.g. "\." matches a ".", not any
+literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
character; "\\" matches a "\"). A series of characters matches that
series of characters in the target string, so the pattern C<blurfl>
would match "blurfl" in the target string.
used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
of octal digits, matches the character whose ASCII value is I<nnn>.
-Similarly, \xI<nn>, where I<nn> are hexidecimal digits, matches the
+Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
character whose ASCII value is I<nn>. The expression \cI<x> matches the
ASCII character control-I<x>. Finally, the "." metacharacter matches any
character except "\n" (unless you use C</s>).
opening parenthesis. Note that a backreference matches whatever
actually matched the subpattern in the string being examined, not the
rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
-match "0x1234 0x4321",but not "0x1234 01234", since subpattern 1
+match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1
actually matched "0x", even though the rule C<0|0x> could
potentially match the leading 0 in the second number.
This is grandfathered for the RHS of a substitute to avoid shocking the
B<sed> addicts, but it's a dirty habit to get into. That's because in
-PerlThink, the right-hand side of a C<s///> is a double-quoted string. C<\1> in
+PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
the usual double-quoted string means a control-A. The customary Unix
meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
of doing that, you get yourself into trouble if you then add an C</e>
C<${1}000>. Basically, the operation of interpolation should not be confused
with the operation of matching a backreference. Certainly they mean two
different things on the I<left> side of the C<s///>.
+
+=head2 SEE ALSO
+
+"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.
projects / p5sagit/p5-mst-13.2.git / blobdiff