=item m
Treat string as multiple lines. That is, change "^" and "$" from matching
-at only the very start or end of the string to the start or end of any
+the start or end of the string to matching the start or end of any
line anywhere within the string.
=item s
=back
These are usually written as "the C</x> modifier", even though the delimiter
-in question might not actually be a slash. In fact, any of these
+in question might not really be a slash. Any of these
modifiers may also be embedded within the regular expression itself using
-the new C<(?...)> construct. See below.
+the C<(?...)> construct. See below.
The C</x> modifier itself needs a little more explanation. It tells
the regular expression parser to ignore whitespace that is neither
your regular expression into (slightly) more readable parts. The C<#>
character is also treated as a metacharacter introducing a comment,
just as in ordinary Perl code. This also means that if you want real
-whitespace or C<#> characters in the pattern (outside of a character
+whitespace or C<#> characters in the pattern (outside a character
class, where they are unaffected by C</x>), that you'll either have to
escape them or encode them using octal or hex escapes. Taken together,
these features go a long way towards making Perl's regular expressions
=head2 Regular Expressions
The patterns used in Perl pattern matching derive from supplied in
-the Version 8 regex routines. (In fact, the routines are derived
+the Version 8 regex routines. (The routines are derived
(distantly) from Henry Spencer's freely redistributable reimplementation
of the V8 routines.) See L<Version 8 Regular Expressions> for
details.
() Grouping
[] Character class
-By default, the "^" character is guaranteed to match at only the
-beginning of the string, the "$" character at only the end (or before the
-newline at the end) and Perl does certain optimizations with the
+By default, the "^" character is guaranteed to match only the
+beginning of the string, the "$" character only the end (or before the
+newline at the end), and Perl does certain optimizations with the
assumption that the string contains only one line. Embedded newlines
will not be matched by "^" or "$". You may, however, wish to treat a
string as a multi-line buffer, such that the "^" will match after any
on the pattern match operator. (Older programs did this by setting C<$*>,
but this practice is now deprecated.)
-To facilitate multi-line substitutions, the "." character never matches a
+To simplify multi-line substitutions, the "." character never matches a
newline unless you use the C</s> modifier, which in effect tells Perl to pretend
the string is a single line--even if it isn't. The C</s> modifier also
overrides the setting of C<$*>, in case you have some (badly behaved) older
\C Match a single C char (octet) even under utf8.
A C<\w> matches a single alphanumeric character, not a whole 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). See L<utf8> for details
-about C<\pP>, C<\PP>, and C<\X>.
+Use C<\w+> to match a string of Perl-identifier characters (which isn't
+the same as matching an English word). 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). See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>.
+
+The POSIX character class syntax
+
+ [:class:]
+
+is also available. The available classes and their \-equivalents
+(if any) are as follows:
+
+ alpha
+ alnum
+ ascii
+ cntrl
+ digit \d
+ graph
+ lower
+ print
+ punct
+ space \s
+ upper
+ word \w
+ xdigit
+
+Note that the [] are part of the [::] construct, not part of the whole
+character class. For example:
+
+ [01[:alpha:]%]
+
+matches one, zero, any alphabetic character, and the percentage sign.
+
+The exact meanings of the above classes depend from many things:
+if the C<utf8> pragma is used, the following equivalenced to Unicode
+\p{} constructs hold:
+
+ alpha IsAlpha
+ alnum IsAlnum
+ ascii IsASCII
+ cntrl IsCntrl
+ digit IsDigit
+ graph IsGraph
+ lower IsLower
+ print IsPrint
+ punct IsPunct
+ space IsSpace
+ upper IsUpper
+ word IsWord
+ xdigit IsXDigit
+
+For example, [:lower:] and \p{IsLower} are equivalent.
+
+If the C<utf8> pragma is not used but the C<locale> pragma is, the
+classes correlate with the isalpha(3) interface (except for `word',
+which is a Perl extension).
+
+The assumedly non-obviously named classes are:
+
+=over 4
+
+=item cntrl
+
+ Any control character. Usually characters that don't produce
+ output as such but instead control the terminal somehow:
+ for example newline and backspace are control characters.
+
+=item graph
+
+ Any alphanumeric or punctuation character.
+
+=item print
+
+ Any alphanumeric or punctuation character or space.
+
+=item punct
+
+ Any punctuation character.
+
+=item xdigit
+
+ Any hexadecimal digit. Though this may feel silly
+ (/0-9a-f/i would work just fine) it is included
+ for completeness.
+
+=item
+
+=back
+
+You can negate the [::] character classes by prefixing the class name
+with a '^'. This is a Perl extension. For example:
+
+ ^digit \D \P{IsDigit}
+ ^space \S \P{IsSpace}
+ ^word \W \P{IsWord}
+
+The POSIX character classes [.cc.] and [=cc=] are B<not> supported
+and trying to use them will cause an error.
Perl defines the following zero-width assertions:
\z Match only at end of string
\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
+A word boundary (C<\b>) is a spot between two characters
that 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
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 to create a capture
-buffer, \E<lt>digitE<gt> matches the digit'th substring. Outside
-of the pattern, always use "$" instead of "\" 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 $E<lt>digitE<gt> (and C<$`>,
-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 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 captured substrings, the variables $10, $11, ... refer to
-the corresponding substring. Within the pattern, \10, \11, etc.
-refer back to substrings if there have been at least that many left
-parentheses before the backreference. Otherwise (for backward
-compatibility) \10 is the same as \010, a backspace, and \11 the
-same as \011, a tab. And so on. (\1 through \9 are always
-backreferences.)
-
-C<$+> returns whatever the last bracket match matched. C<$&> returns the
-entire matched string. (C<$0> used to return the same thing, but not any
-more.) C<$`> returns everything before the matched string. C<$'> returns
-everything after the matched string. Examples:
+
+The bracketing construct C<( ... )> creates capture buffers. To
+refer to the digit'th buffer use \E<lt>digitE<gt> within the
+match. Outside the match use "$" instead of "\". (The
+\E<lt>digitE<gt> notation works in certain circumstances outside
+the match. See the warning below about \1 vs $1 for details.)
+Referring back to another part of the match is called a
+I<backreference>.
+
+There is no limit to the number of captured substrings that you may
+use. However Perl also uses \10, \11, etc. as aliases for \010,
+\011, etc. (Recall that 0 means octal, so \011 is the 9'th ASCII
+character, a tab.) Perl resolves this ambiguity by interpreting
+\10 as a backreference only if at least 10 left parentheses have
+opened before it. Likewise \11 is a backreference only if at least
+11 left parentheses have opened before it. And so on. \1 through
+\9 are always interpreted as backreferences."
+
+Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
- if (/Time: (..):(..):(..)/) {
+ if (/(.)\1/) { # find first doubled char
+ print "'$1' is the first doubled character\n";
+ }
+
+ if (/Time: (..):(..):(..)/) { # parse out values
$hours = $1;
$minutes = $2;
$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 pattern that contains capturing parentheses. But if you never
-use $&, etc., in your script, then patterns 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. As of 5.005, $& is not so costly as the other two.
+
+Several special variables also refer back to portions of the previous
+match. C<$+> returns whatever the last bracket match matched.
+C<$&> returns the entire matched string. (At one point C<$0> did
+also, but now it returns the name of the program.) C<$`> returns
+everything before the matched string. And C<$'> returns everything
+after the matched string.
+
+The numbered variables ($1, $2, $3, etc.) and the related punctuation
+set (C<<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped
+until the end of the enclosing block or until the next successful
+match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
+
+B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or
+C<$'> anywhere in the program, it has to provide them for every
+pattern match. This may substantially slow your program. Perl
+uses the same mechanism to produce $1, $2, etc, so you also pay a
+price for each pattern that contains capturing parentheses. (To
+avoid this cost while retaining the grouping behaviour, use the
+extended regular expression C<(?: ... )> instead.) But if you never
+use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing
+parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`>
+if you can, but if you can't (and some algorithms really appreciate
+them), once you've used them once, use them at will, because you've
+already paid the price. As of 5.005, C<$&> is not so costly as the
+other two.
Backslashed metacharacters in Perl are alphanumeric, such as C<\b>,
C<\w>, C<\n>. Unlike some other regular expression languages, there
$pattern =~ s/(\W)/\\$1/g;
-In modern days, it is more common to see either the quotemeta()
-function or the C<\Q> metaquoting escape sequence used to disable
-all metacharacters' special meanings like this:
+Today it is more common to use the quotemeta() function or the C<\Q>
+metaquoting escape sequence to disable all metacharacters' special
+meanings like this:
/$unquoted\Q$quoted\E$unquoted/
=head2 Extended Patterns
-For those situations where simple regular expression patterns are
-not enough, Perl defines a consistent extension syntax for venturing
-beyond simple patterns such as are found in standard tools like
-B<awk> and B<lex>. That 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.
+Perl also defines a consistent extension syntax for features not
+found in standard tools like B<awk> and B<lex>. The syntax is a
+pair of parentheses with a question mark as the first thing within
+the parentheses. The character after the question mark indicates
+the extension.
-Many extensions are already supported, some for almost five years
-now. Other, more exotic forms are very new, and should be considered
-highly experimental, and are so marked.
+The stability of these extensions varies widely. Some have been
+part of the core language for many years. Others are experimental
+and may change without warning or be completely removed. Check
+the documentation on an individual feature to verify its current
+status.
-A question mark was chosen for this and for the new minimal-matching
-construct because 1) question mark is pretty rare in older regular
-expressions, and 2) whenever you see one, you should stop and "question"
-exactly what is going on. That's psychology...
+A question mark was chosen for this and for the minimal-matching
+construct because 1) question marks are rare in older regular
+expressions, and 2) whenever you see one, you should stop and
+"question" exactly what is going on. That's psychology...
=over 10
=item C<(?#text)>
-A comment. The text is ignored. If the C</x> modifier is used to enable
+A comment. The text is ignored. If the C</x> modifier enables
whitespace formatting, a simple C<#> will suffice. Note that Perl closes
the comment as soon as it sees a C<)>, so there is no way to put a literal
C<)> in the comment.
( (?i) blah ) \s+ \1
will match a repeated (I<including the case>!) word C<blah> in any
-case, assuming C<x> modifier, and no C<i> modifier outside of this
+case, assuming C<x> modifier, and no C<i> modifier outside this
group.
=item C<(?:pattern)>
/(?s-i:more.*than).*million/i
-is equivalent to more verbose
+is equivalent to the more verbose
/(?:(?s-i)more.*than).*million/i
>x;
will set C<$res = 4>. Note that after the match, $cnt returns to the globally
-introduced value, since the scopes which restrict C<local> operators
+introduced value, because the scopes that restrict C<local> operators
are unwound.
This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)>
variables contain results of C<qr//> operator (see
L<perlop/"qr/STRING/imosx">).
-This restriction is due to the wide-spread and remarkably convenient
+This restriction is because of the wide-spread and remarkably convenient
custom of using run-time determined strings as patterns. For example:
$re = <>;
chomp $re;
$string =~ /$re/;
-Prior to the execution of code in a pattern, this was completely
-safe from a security point of view, although it could of course
-raise an exception from an illegal pattern. If you turn on the
-C<use re 'eval'>, though, it is no longer secure, so you should
-only do so if you are also using taint checking. Better yet, use
-the carefully constrained evaluation within a Safe module. See
-L<perlsec> for details about both these mechanisms.
+Before Perl knew how to execute interpolated code within a pattern,
+this operation was completely safe from a security point of view,
+although it could raise an exception from an illegal pattern. If
+you turn on the C<use re 'eval'>, though, it is no longer secure,
+so you should only do so if you are also using taint checking.
+Better yet, use the carefully constrained evaluation within a Safe
+module. See L<perlsec> for details about both these mechanisms.
=item C<(?p{ code })>
An "independent" subexpression, one which matches the substring
that a I<standalone> C<pattern> would match if anchored at the given
-position -- but it matches no more than this substring. This
+position--but it matches no more than this substring. This
construct is useful for optimizations of what would otherwise be
"eternal" matches, because it will not backtrack (see L<"Backtracking">).
above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several
seconds, but that each extra letter doubles this time. This
exponential performance will make it appear that your program has
-hung. However, a tiny modification of this pattern
+hung. However, a tiny change to this pattern
m{ \(
(
got <d is under the bar in the >
That's because C<.*> was greedy, so you get everything between the
-I<first> "foo" and the I<last> "bar". In this case, it's more effective
+I<first> "foo" and the I<last> "bar". Here it's more effective
to use minimal matching to make sure you get the text between a "foo"
and the first "bar" thereafter.
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.
+
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
+try to match C<(?!123> with "123", which 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.
The pattern really, I<really> wants to succeed, so it uses the
standard pattern 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.
+"123". It's "C123", which suffices.
-We can deal with this by using both an assertion and a negation. We'll
-say that the first part in $1 must be followed by a digit, and in fact, it
-must also be followed by something that's not "123". Remember that the
-look-aheads are zero-width expressions--they only look, but don't consume
-any of the string in their match. So rewriting this way produces what
+We can deal with this by using both an assertion and a negation.
+We'll say that the first part in $1 must be followed both by a digit
+and by something that's not "123". Remember that the look-aheads
+are zero-width expressions--they only look, but don't consume any
+of the string in their match. So rewriting this way produces what
you'd expect; that is, case 5 will fail, but case 6 succeeds:
print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
is not a zero-width assertion, but a one-width assertion.
B<WARNING>: particularly complicated regular expressions can take
-exponential time to solve due to the immense number of possible
+exponential time to solve because of the immense number of possible
ways they can use backtracking to try match. For example, this will
-take a very long time to run
+take a painfully long time to run
/((a{0,5}){0,5}){0,5}/
-And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
-it would take literally forever--or until you ran out of stack space.
+And if you used C<*>'s instead of limiting it to 0 through 5 matches,
+then it would take forever--or until you ran out of stack space.
A powerful tool for optimizing such beasts is "independent" groups,
which do not backtrace (see L<C<(?E<gt>pattern)>>). Note also that
zero-length look-ahead/look-behind assertions will not backtrace to make
-the tail match, since they are in "logical" context: only the fact
-whether they match or not is considered relevant. For an example
+the tail match, since they are in "logical" context: only
+whether they match is considered relevant. For an example
where side-effects of a look-ahead I<might> have influenced the
following match, see L<C<(?E<gt>pattern)>>.
You can specify a character class, by enclosing a list of characters
in C<[]>, which will match any one character from the list. If the
first character after the "[" is "^", the class matches any character not
-in the list. Within a list, the "-" character is used to specify a
+in the list. Within a list, the "-" character specifies a
range, so that C<a-z> represents all characters between "a" and "z",
inclusive. If you want "-" itself to be a member of a class, put it
at the start or end of the list, or escape it with a backslash. (The
first alternative includes everything from the last pattern delimiter
("(", "[", or the beginning of the pattern) up to the first "|", and
the last alternative contains everything from the last "|" to the next
-pattern delimiter. For this reason, it's common practice to include
-alternatives in parentheses, to minimize confusion about where they
+pattern delimiter. That's why it's common practice to include
+alternatives in parentheses: to minimize confusion about where they
start and end.
Alternatives are tried from left to right, so the first
Also remember that "|" is interpreted as a literal within square brackets,
so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>.
-Within a pattern, you may designate subpatterns for later reference by
-enclosing them in parentheses, and you may refer back to the I<n>th
-subpattern later in the pattern using the metacharacter \I<n>.
-Subpatterns are numbered based on the left to right order of their
-opening parenthesis. 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", because subpattern 1
-actually matched "0x", even though the rule C<0|0x> could
-potentially match the leading 0 in the second number.
+Within a pattern, you may designate subpatterns for later reference
+by enclosing them in parentheses, and you may refer back to the
+I<n>th subpattern later in the pattern using the metacharacter
+\I<n>. Subpatterns are numbered based on the left to right order
+of their opening parenthesis. 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", because subpattern
+1 matched "0x", even though the rule C<0|0x> could potentially match
+the leading 0 in the second number.
=head2 Warning on \1 vs $1
s/(\d+)/\1000/;
You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
-C<${1}000>. Basically, the operation of interpolation should not be confused
+C<${1}000>. 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///>.
The C<o?> can match at the beginning of C<'foo'>, and since the position
in the string is not moved by the match, C<o?> would match again and again
-due to the C<*> modifier. Another common way to create a similar cycle
+because of the C<*> modifier. Another common way to create a similar cycle
is with the looping modifier C<//g>:
@matches = ( 'foo' =~ m{ o? }xg );
or the loop implied by split().
However, long experience has shown that many programming tasks may
-be significantly simplified by using repeated subexpressions which
-may match zero-length substrings, with a simple example being:
+be significantly simplified by using repeated subexpressions that
+may match zero-length substrings. Here's a simple example being:
@chars = split //, $string; # // is not magic in split
($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /
L<perlop/"Gory details of parsing quoted constructs">.
+L<perlfaq6>.
+
L<perlfunc/pos>.
L<perllocale>.
-I<Mastering Regular Expressions> by Jeffrey Friedl.
+I<Mastering Regular Expressions> by Jeffrey Friedl, published
+by O'Reilly and Associates.
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