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Perl Regular Expressions - detailed manual
NAME
perlre - Perl regular expressions

DESCRIPTION
This page describes the syntax of regular expressions in
Perl.  For a description of how to use regular expressions
in matching operations, plus various examples of the same,
see discussions of `m//', `s///', `qr//' and `??' in the
Regexp Quote-Like Operators entry in the perlop manpage.

Matching operations can have various modifiers.	Modifiers
that relate to the interpretation of the regular expres­
sion inside are listed below.  Modifiers that alter the
way a regular expression is used by Perl are detailed in
the Regexp Quote-Like Operators entry in the perlop man­
page and the Gory details of parsing quoted constructs
entry in the perlop manpage.

i   Do case-insensitive pattern matching.

	   If `use locale' is in effect, the case map is taken
	   from the current locale.  See the perllocale manpage.

m   Treat string as multiple lines.  That is, change "^"
	   and "$" from matching the start or end of the string
	   to matching the start or end of any line anywhere
	   within the string.

s   Treat string as single line.	 That is, change "." to
	   match any character whatsoever, even a newline, which
	   normally it would not match.

	   The `/s' and `/m' modifiers both override the `$*'
	   setting.  That is, no matter what `$*' contains, `/s'
	   without `/m' will force "^" to match only at the
	   beginning of the string and "$" to match only at the
	   end (or just before a newline at the end) of the
	   string.  Together, as /ms, they let the "." match any
	   character whatsoever, while yet allowing "^" and "$"
	   to match, respectively, just after and just before
	   newlines within the string.

x   Extend your pattern's legibility by permitting whites­
	   pace and comments.

These are usually written as "the `/x' modifier", even
though the delimiter in question might not really be a
slash.  Any of these modifiers may also be embedded within
the regular expression itself using the `(?...)' con­
struct.	See below.

The `/x' modifier itself needs a little more explanation.
It tells the regular expression parser to ignore whites­
pace that is neither backslashed nor within a character
class.  You can use this to break up your regular expres­
sion into (slightly) more readable parts.  The `#' charac­
ter is also treated as a metacharacter introducing a com­
ment, just as in ordinary Perl code.  This also means that
if you want real whitespace or `#' characters in the pat­
tern (outside a character class, where they are unaffected
by `/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 more readable.  Note that you have to be care­
ful not to include the pattern delimiter in the com­
ment--perl has no way of knowing you did not intend to
close the pattern early.	 See the C-comment deletion code
in the perlop manpage.

Regular Expressions

The patterns used in Perl pattern matching derive from
supplied in the Version 8 regex routines.  (The routines
are derived (distantly) from Henry Spencer's freely redis­
tributable reimplementation of the V8 routines.)	 See the
Version 8 Regular Expressions entry elsewhere in this doc­
ument for details.

In particular the following metacharacters have their
standard egrep-ish meanings:

	   \   Quote the next metacharacter
	   ^   Match the beginning of the line
	   .   Match any character (except newline)
	   $   Match the end of the line (or before newline at the end)
	   |   Alternation
	   ()  Grouping
	   []  Character class

By default, the "^" character is guaranteed to match only
the beginning of the string, the "$" character only the
end (or before the newline at the end), and Perl does cer­
tain optimizations with the assumption that the string
contains only one line.	Embedded newlines will not be
matched by "^" or "$".  You may, however, wish to treat a
string as a multi-line buffer, such that the "^" will
match after any newline within the string, and "$" will
match before any newline.  At the cost of a little more
overhead, you can do this by using the /m modifier on the
pattern match operator.	(Older programs did this by set­
ting `$*', but this practice is now deprecated.)

To simplify multi-line substitutions, the "." character
never matches a newline unless you use the `/s' modifier,
which in effect tells Perl to pretend the string is a sin­
gle line--even if it isn't.  The `/s' modifier also over­
rides the setting of `$*', in case you have some (badly
behaved) older code that sets it in another module.

The following standard quantifiers are recognized:

	   *	  Match 0 or more times
	   +	  Match 1 or more times
	   ?	  Match 1 or 0 times
	   {n}	  Match exactly n times
	   {n,}	  Match at least n times
	   {n,m}  Match at least n but not more than m times

(If a curly bracket occurs in any other context, it is
treated as a regular character.)	 The "*" modifier is
equivalent to `{0,}', the "+" modifier to `{1,}', and the
"?" modifier to `{0,1}'.	 n and m are limited to integral
values less than a preset limit defined when perl is
built.  This is usually 32766 on the most common plat­
forms.  The actual limit can be seen in the error message
generated by code such as this:

	   $_ **= $_ , / {$_} / for 2 .. 42;

By default, a quantified subpattern is "greedy", that is,
it will match as many times as possible (given a particu­
lar 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
	   ??	  Match 0 or 1 time
	   {n}?	  Match exactly n times
	   {n,}?  Match at least n times
	   {n,m}? Match at least n but not more than m times

Because patterns are processed as double quoted strings,
the following also work:
	   \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
	   \x{263a}    wide hex char	     (Unicode SMILEY)
	   \c[	control char
	   \N{name}    named char
	   \l	lowercase next char (think vi)
	   \u	uppercase next char (think vi)
	   \L	lowercase till \E (think vi)
	   \U	uppercase till \E (think vi)
	   \E	end case modification (think vi)
	   \Q	quote (disable) pattern metacharacters till \E

If `use locale' is in effect, the case map used by `\l',
`\L', `\u' and `\U' is taken from the current locale.  See
the perllocale manpage.	For documentation of `\N{name}',
see the charnames manpage.

You cannot include a literal `$' or `@' within a `\Q'
sequence.  An unescaped `$' or `@' interpolates the corre­
sponding variable, while escaping will cause the literal
string `\$' to be matched.  You'll need to write something
like `m/\Quser\E\@\Qhost/'.

In addition, Perl defines the following:

	   \w  Match a "word" character (alphanumeric plus "_")
	   \W  Match a non-word character
	   \s  Match a whitespace character
	   \S  Match a non-whitespace character
	   \d  Match a digit character
	   \D  Match a non-digit character
	   \pP Match P, named property.	 Use \p{Prop} for longer names.
	   \PP Match non-P
	   \X  Match eXtended Unicode "combining character sequence",
	equivalent to C<(?:\PM\pM*)>
	   \C  Match a single C char (octet) even under utf8.

A `\w' matches a single alphanumeric character, not a
whole word.  Use `\w+' to match a string of Perl-identi­
fier characters (which isn't the same as matching an
English word).  If `use locale' is in effect, the list of
alphabetic characters generated by `\w' is taken from the
current locale.	See the perllocale manpage.  You may use
`\w', `\W', `\s', `\S', `\d', and `\D' within character
classes, but if you try to use them as endpoints of a
range, that's not a range, the "-" is understood liter­
ally.  See the utf8 manpage for details about `\pP',
`\PP', and `\X'.

The POSIX character class syntax

	   [:class:]

is also available.  The available classes and their back­
slash equivalents (if available) are as follows:

	   alpha
	   alnum
	   ascii
	   cntrl
	   digit\d
	   graph
	   lower
	   print
	   punct
	   space\s
	   upper
	   word	\w
	   xdigit

For example use `[:upper:]' to match all the uppercase
characters.  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 per­
centage sign.

If the `utf8' pragma is used, the following equivalences
to Unicode \p{} constructs hold:

	   alphaIsAlpha
	   alnumIsAlnum
	   asciiIsASCII
	   cntrlIsCntrl
	   digitIsDigit
	   graphIsGraph
	   lowerIsLower
	   printIsPrint
	   punctIsPunct
	   spaceIsSpace
	   upperIsUpper
	   word	IsWord
	   xdigit      IsXDigit

For example `[:lower:]' and `\p{IsLower}' are equivalent.

If the `utf8' pragma is not used but the `locale' pragma
is, the classes correlate with the isalpha(3) interface
(except for `word', which is a Perl extension, mirroring
`\w').

The assumedly non-obviously named classes are:

cntrl
	   Any control character.  Usually characters that don't
	   produce output as such but instead control the termi­
	   nal somehow: for example newline and backspace are
	   control characters.	All characters with ord() less
	   than 32 are most often classified as control charac­
	   ters.

graph
	   Any alphanumeric or punctuation character.

print
	   Any alphanumeric or punctuation character or space.

punct
	   Any punctuation character.

xdigit
	   Any hexadecimal digit.  Though this may feel silly
	   (/0-9a-f/i would work just fine) it is included for
	   completeness.

You can negate the [::] character classes by prefixing the
class name with a '^'. This is a Perl extension. For example:

	   POSIXtrad. Perl  utf8 Perl

	   [:^digit:]	   \D	   \P{IsDigit}
	   [:^space:]	   \S	   \P{IsSpace}
	   [:^word:]	   \W	   \P{IsWord}

The POSIX character classes [.cc.] and [=cc=] are recog­
nized but not supported and trying to use them will cause
an error.

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
	   \z  Match only at end of string
	   \G  Match only at pos() (e.g. at the end-of-match position
	of prior m//g)

A word boundary (`\b') is a spot between two characters
that has a `\w' on one side of it and a `\W' on the other
side of it (in either order), counting the imaginary char­
acters off the beginning and end of the string as matching
a `\W'.	(Within character classes `\b' represents
backspace rather than a word boundary, just as it normally
does in any double-quoted string.)  The `\A' and `\Z' are
just like "^" and "$", except that they won't match multi­
ple times when the `/m' modifier is used, while "^" and
"$" will match at every internal line boundary.	To match
the actual end of the string and not ignore an optional
trailing newline, use `\z'.

The `\G' assertion can be used to chain global matches
(using `m//g'), as described in the Regexp Quote-Like
Operators entry in the perlop manpage.  It is also useful
when writing `lex'-like scanners, when you have several
patterns that you want to match against consequent sub­
strings of your string, see the previous reference.  The
actual location where `\G' will match can also be influ­
enced by using `pos()' as an lvalue.  See the pos entry in
the perlfunc manpage.

The bracketing construct `( ... )' creates capture
buffers.	 To refer to the digit'th buffer use \<digit>
within the match.  Outside the match use "$" instead of
"\".  (The \<digit> notation works in certain circum­
stances outside the match.  See the warning below about \1
vs $1 for details.)  Referring back to another part of the
match is called a 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 backref­
erence 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 backrefer­
ences."

Here are some examples:

	   s/^([^ ]*) *([^ ]*)/$2 $1/;	   # swap first two words

	    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;
	   }

Several special variables also refer back to portions of
the previous match.  `$+' returns whatever the last
bracket match matched.  `$&' returns the entire matched
string.	(At one point `$0' did also, but now it returns
the name of the program.)  `$`' returns everything before
the matched string.  And `$'' returns everything after the
matched string.

The numbered variables ($1, $2, $3, etc.) and the related
punctuation set (`<$+', `$&', `$`', and `$'') are all
dynamically scoped until the end of the enclosing block or
until the next successful match, whichever comes first.
(See the Compound Statements entry in the perlsyn man­
page.)

A WARNING: Once Perl sees that you need one of `$&', `$`',
or `$'' 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, $2,
etc, so you also pay a price for each pattern that con­
tains capturing parentheses.  (To avoid this cost while
retaining the grouping behaviour, use the extended regular
expression `(?: ... )' instead.)	 But if you never use
`$&', `$`' or `$'', then patterns without capturing paren­
theses will not 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.

Backslashed metacharacters in Perl are alphanumeric, such
as `\b', `\w', `\n'.  Unlike some other regular expression
languages, there are no backslashed symbols that aren't
alphanumeric.  So anything that looks like \\, \(, \), \<,
\>, \{, or \} is always interpreted as a literal charac­
ter, not a metacharacter.  This was once used in a common
idiom to disable or quote the special meanings of regular
expression metacharacters in a string that you want to use
for a pattern. Simply quote all non-alphanumeric characters:

	   $pattern =~ s/(\W)/\\$1/g;

Today it is more common to use the quotemeta() function or
the `\Q' metaquoting escape sequence to disable all
metacharacters' special meanings like this:

	   /$unquoted\Q$quoted\E$unquoted/

Beware that if you put literal backslashes (those not
inside interpolated variables) between `\Q' and `\E', dou­
ble-quotish backslash interpolation may lead to confusing
results.	 If you need to use literal backslashes within
`\Q...\E', consult the Gory details of parsing quoted con­
structs entry in the perlop manpage.

Extended Patterns

Perl also defines a consistent extension syntax for fea­
tures not found in standard tools like awk and 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.

The stability of these extensions varies widely.	 Some
have been part of the core language for many years.  Oth­
ers are experimental and may change without warning or be
completely removed.  Check the documentation on an indi­
vidual feature to verify its current status.

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...

`(?#text)'
		 A comment.  The text is ignored.  If the `/x'
		 modifier enables whitespace formatting, a simple
		 `#' will suffice.  Note that Perl closes the
		 comment as soon as it sees a `)', so there is no
		 way to put a literal `)' in the comment.

`(?imsx-imsx)'
		 One or more embedded pattern-match modifiers.
		 This is particularly useful for dynamic pat­
		 terns, such as those read in from a configura­
		 tion file, read in as an argument, are specified
		 in a table somewhere, etc.  Consider the case
		 that some of which want to be case sensitive and
		 some do not.  The case insensitive ones need to
		 include merely `(?i)' at the front of the pat­
		 tern.	For example:

		     $pattern = "foobar";
		     if ( /$pattern/i ) { }

		     # more flexible:

		     $pattern = "(?i)foobar";
		     if ( /$pattern/ ) { }

		 Letters after a `-' turn those modifiers off.
		 These modifiers are localized inside an enclos­
		 ing group (if any).  For example,

		     ( (?i) blah ) \s+ \1

		 will match a repeated (including the case!) word
		 `blah' in any case, assuming `x' modifier, and
		 no `i' modifier outside this group.

`(?:pattern)'

`(?imsx-imsx:pattern)'
		 This is for clustering, not capturing; it groups
		 subexpressions like "()", but doesn't make back­
		 references as "()" does.  So

		     @fields = split(/\b(?:a|b|c)\b/)

		 is like

		     @fields = split(/\b(a|b|c)\b/)

		 but doesn't spit out extra fields.  It's also
		 cheaper not to capture characters if you don't
		 need to.

		 Any letters between `?' and `:' act as flags
		 modifiers as with `(?imsx-imsx)'.  For example,

		     /(?s-i:more.*than).*million/i

		 is equivalent to the more verbose

		     /(?:(?s-i)more.*than).*million/i


`(?=pattern)'
		 A zero-width positive look-ahead assertion.  For
		 example, `/\w+(?=\t)/' matches a word followed
		 by a tab, without including the tab in `$&'.

`(?!pattern)'
		 A zero-width negative look-ahead assertion.  For
		 example `/foo(?!bar)/' matches any occurrence of
		 "foo" that isn't followed by "bar".  Note how­
		 ever that look-ahead and look-behind are NOT the
		 same thing.  You cannot use this for look-
		 behind.

		 If you are looking for a "bar" that isn't pre­
		 ceded by a "foo", `/(?!foo)bar/' will not do
		 what you want.	 That's because the `(?!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
		 `/(?!foo)...bar/' for that.   We say "like"
		 because there's the case of your "bar" not hav­
		 ing three characters before it.  You could cover
		 that this way: `/(?:(?!foo)...|^.{0,2})bar/'.
		 Sometimes it's still easier just to say:

		     if (/bar/ && $` !~ /foo$/)

		 For look-behind see below.

`(?<=pattern)'
		 A zero-width positive look-behind assertion.
		 For example, `/(?<=\t)\w+/' matches a word that
		 follows a tab, without including the tab in
		 `$&'.	Works only for fixed-width look-behind.

`(?<!pattern)'
		 A zero-width negative look-behind assertion.
		 For example `/(?<!bar)foo/' matches any occur­
		 rence of "foo" that does not follow "bar".
		 Works only for fixed-width look-behind.

`(?{ code })'
		 WARNING: This extended regular expression fea­
		 ture is considered highly experimental, and may
		 be changed or deleted without notice.

		 This zero-width assertion evaluate any embedded
		 Perl code.  It always succeeds, and its `code'
		 is not interpolated.  Currently, the rules to
		 determine where the `code' ends are somewhat
		 convoluted.

		 The `code' is properly scoped in the following
		 sense: If the assertion is backtracked (compare
		 the section on "Backtracking"), all changes
		 introduced after `local'ization are undone, so
		 that

		   $_ = 'a' x 8;
		   m<
		      (?{ $cnt = 0 })			 # Initialize $cnt.
		      (
			a
			(?{
			    local $cnt = $cnt + 1;	 # Update $cnt, backtracking-safe.
			})
		      )*
		      aaaa
		      (?{ $res = $cnt })		 # On success copy to non-localized
							 # location.
		    >x;

		 will set `$res = 4'.  Note that after the match,
		 $cnt returns to the globally introduced value,
		 because the scopes that restrict `local' opera­
		 tors are unwound.

		 This assertion may be used as a `(?(condi­
		 tion)yes-pattern|no-pattern)' switch.	If not
		 used in this way, the result of evaluation of
		 `code' is put into the special variable `$^R'.
		 This happens immediately, so `$^R' can be used
		 from other `(?{ code })' assertions inside the
		 same regular expression.

		 The assignment to `$^R' above is properly local­
		 ized, so the old value of `$^R' is restored if
		 the assertion is backtracked; compare the sec­
		 tion on "Backtracking".

		 For reasons of security, this construct is for­
		 bidden if the regular expression involves run-
		 time interpolation of variables, unless the per­
		 ilous `use re 'eval'' pragma has been used (see
		 the re manpage), or the variables contain
		 results of `qr//' operator (see the
		 qr/STRING/imosx entry in the perlop manpage).

		 This restriction is because of the wide-spread
		 and remarkably convenient custom of using run-
		 time determined strings as patterns.  For exam­
		 ple:

		     $re = <>;
		     chomp $re;
		     $string =~ /$re/;

		 Before Perl knew how to execute interpolated
		 code within a pattern, this operation was com­
		 pletely safe from a security point of view,
		 although it could raise an exception from an
		 illegal pattern.  If you turn on the `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 con­
		 strained evaluation within a Safe module.  See
		 the perlsec manpage for details about both these
		 mechanisms.

`(??{ code })'
		 WARNING: This extended regular expression fea­
		 ture is considered highly experimental, and may
		 be changed or deleted without notice.	A simpli­
		 fied version of the syntax may be introduced for
		 commonly used idioms.

		 This is a "postponed" regular subexpression.
		 The `code' is evaluated at run time, at the
		 moment this subexpression may match.  The result
		 of evaluation is considered as a regular expres­
		 sion and matched as if it were inserted instead
		 of this construct.

		 The `code' is not interpolated.  As before, the
		 rules to determine where the `code' ends are
		 currently somewhat convoluted.

		 The following pattern matches a parenthesized
		 group:

		   $re = qr{
			      \(
			      (?:
				 (?> [^()]+ )	 # Non-parens without backtracking
			|
				 (??{ $re })	 # Group with matching parens
			      )*
			      \)
			   }x;


`(?>pattern)'
		 WARNING: This extended regular expression fea­
		 ture is considered highly experimental, and may
		 be changed or deleted without notice.

		 An "independent" subexpression, one which
		 matches the substring that a standalone `pat­
		 tern' would match if anchored at the given posi­
		 tion, and it matches nothing other than this
		 substring.  This construct is useful for opti­
		 mizations of what would otherwise be "eternal"
		 matches, because it will not backtrack (see the
		 section on "Backtracking").  It may also be use­
		 ful in places where the "grab all you can, and
		 do not give anything back" semantic is desir­
		 able.

		 For example: `^(?>a*)ab' will never match, since
		 `(?>a*)' (anchored at the beginning of string,
		 as above) will match all characters `a' at the
		 beginning of string, leaving no `a' for `ab' to
		 match.	 In contrast, `a*ab' will match the same
		 as `a+b', since the match of the subgroup `a*'
		 is influenced by the following group `ab' (see
		 the section on "Backtracking").  In particular,
		 `a*' inside `a*ab' will match fewer characters
		 than a standalone `a*', since this makes the
		 tail match.

		 An effect similar to `(?>pattern)' may be
		 achieved by writing `(?=(pattern))\1'.	 This
		 matches the same substring as a standalone `a+',
		 and the following `\1' eats the matched string;
		 it therefore makes a zero-length assertion into
		 an analogue of `(?>...)'.  (The difference
		 between these two constructs is that the second
		 one uses a capturing group, thus shifting ordi­
		 nals of backreferences in the rest of a regular
		 expression.)

		 Consider this pattern:

		     m{ \(
			   (
			     [^()]+		 # x+
			   |
			     \( [^()]* \)
			   )+
			\)
		      }x

		 That will efficiently match a nonempty group
		 with matching parentheses two levels deep or
		 less.	However, if there is no such group, it
		 will take virtually forever on a long string.
		 That's because there are so many different ways
		 to split a long string into several substrings.
		 This is what `(.+)+' is doing, and `(.+)+' is
		 similar to a subpattern of the above pattern.
		 Consider how the pattern above detects no-match
		 on `((()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
		 change to this pattern

		     m{ \(
			   (
			     (?> [^()]+ )	 # change x+ above to (?> x+ )
			   |
			     \( [^()]* \)
			   )+
			\)
		      }x

		 which uses `(?>...)' matches exactly when the
		 one above does (verifying this yourself would be
		 a productive exercise), but finishes in a fourth
		 the time when used on a similar string with
		 1000000 `a's.	Be aware, however, that this pat­
		 tern currently triggers a warning message under
		 the `use warnings' pragma or -w switch saying it
		 `"matches the null string many times"'):

		 On simple groups, such as the pattern `(?>
		 [^()]+ )', a comparable effect may be achieved
		 by negative look-ahead, as in `[^()]+ (?! [^()]
		 )'.  This was only 4 times slower on a string
		 with 1000000 `a's.

		 The "grab all you can, and do not give anything
		 back" semantic is desirable in many situations
		 where on the first sight a simple `()*' looks
		 like the correct solution.  Suppose we parse
		 text with comments being delimited by `#' fol­
		 lowed by some optional (horizontal) whitespace.
		 Contrary to its appearance, `#[ \t]*' is not the
		 correct subexpression to match the comment
		 delimiter, because it may "give up" some whites­
		 pace if the remainder of the pattern can be made
		 to match that way.  The correct answer is either
		 one of these:

		     (?>#[ \t]*)
		     #[ \t]*(?![ \t])

		 For example, to grab non-empty comments into $1,
		 one should use either one of these:

		     / (?> \# [ \t]* ) (	.+ ) /x;
		     /	   \# [ \t]*   ( [^ \t] .* ) /x;

		 Which one you pick depends on which of these
		 expressions better reflects the above specifica­
		 tion of comments.

`(?(condition)yes-pattern|no-pattern)'

`(?(condition)yes-pattern)'
		 WARNING: This extended regular expression fea­
		 ture is considered highly experimental, and may
		 be changed or deleted without notice.

		 Conditional expression.  `(condition)' should be
		 either an integer in parentheses (which is valid
		 if the corresponding pair of parentheses
		 matched), or look-ahead/look-behind/evaluate
		 zero-width assertion.

		 For example:

		     m{ ( \( )?
			[^()]+
			(?(1) \) )
		      }x

		 matches a chunk of non-parentheses, possibly
		 included in parentheses themselves.

Backtracking

This section presents an abstract approximation of
regular expression behavior.  For a more rigorous (and
complicated) view of the rules involved in selecting a
match among possible alternatives, see the Combining
pieces together entry elsewhere in this document.

A fundamental feature of regular expression matching
involves the notion called backtracking, which is cur­
rently used (when needed) by all regular expression quan­
tifiers, namely `*', `*?', `+', `+?', `{n,m}', and
`{n,m}?'.  Backtracking is often optimized internally, but
the general principle outlined here is valid.

For a regular expression to match, the entire regular
expression must match, not just part of it.  So if the
beginning of a pattern containing a quantifier succeeds in
a way that causes later parts in the pattern to fail, the
matching engine backs up and recalculates the beginning
part--that's why it's called backtracking.

Here is an example of backtracking:  Let's say you want to
find the word following "foo" in the string "Food is on
the foo table.":

	   $_ = "Food is on the foo table.";
	   if ( /\b(foo)\s+(\w+)/i ) {
	print "$2 follows $1.\n";
	   }

When the match runs, the first part of the regular expres­
sion (`\b(foo)') finds a possible match right at the
beginning of the string, and loads up $1 with "Foo".  How­
ever, 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 the tentative match.	This time it goes
all the way until the next occurrence of "foo". The com­
plete regular expression matches this time, and you get
the expected output of "table follows foo."

Sometimes minimal matching can help a lot.  Imagine you'd
like to match everything between "foo" and "bar".  Ini­
tially, you write something like this:

	   $_ =	 "The food is under the bar in the barn.";
	   if ( /foo(.*)bar/ ) {
	print "got <$1>\n";
	   }

Which perhaps unexpectedly yields:

	 got <d is under the bar in the >

That's because `.*' was greedy, so you get everything
between the first "foo" and the 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.

	   if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
	 got <d is under the >

Here's another example: let's say you'd like to match a
number at the end of a string, and you also want to keep
the preceding part the match.  So you write this:

	   $_ = "I have 2 numbers: 53147";
	   if ( /(.*)(\d*)/ ) {				# Wrong!
	print "Beginning is <$1>, number is <$2>.\n";
	   }

That won't work at all, because `.*' was greedy and gob­
bled up the whole string. As `\d*' can match on an empty
string the complete regular expression matched success­
fully.

	   Beginning is <I have 2 numbers: 53147>, number is <>.

Here are some variants, most of which don't work:

	   $_ = "I have 2 numbers: 53147";
	   @pats = qw{
	(.*)(\d*)
	(.*)(\d+)
	(.*?)(\d*)
	(.*?)(\d+)
	(.*)(\d+)$
	(.*?)(\d+)$
	(.*)\b(\d+)$
	(.*\D)(\d+)$
	   };

	   for $pat (@pats) {
	printf "%-12s ", $pat;
	if ( /$pat/ ) {
		   print "<$1> <$2>\n";
	} else {
		   print "FAIL\n";
	}
	   }

That will print out:

	   (.*)(\d*)	<I have 2 numbers: 53147> <>
	   (.*)(\d+)	<I have 2 numbers: 5314> <7>
	   (.*?)(\d*)	<> <>
	   (.*?)(\d+)	<I have > <2>
	   (.*)(\d+)$	<I have 2 numbers: 5314> <7>
	   (.*?)(\d+)$	<I have 2 numbers: > <53147>
	   (.*)\b(\d+)$ <I have 2 numbers: > <53147>
	   (.*\D)(\d+)$ <I have 2 numbers: > <53147>

As you see, this can be a bit tricky.  It's important to
realize that a 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 to know which variety of success you will
achieve.

When using look-ahead assertions and negations, this can
all get even tricker.  Imagine you'd like to find a
sequence of non-digits not followed by "123".  You might
try to write that as

	   $_ = "ABC123";
	   if ( /^\D*(?!123)/ ) {	# Wrong!
	print "Yup, no 123 in $_\n";
	   }

But that isn't going to match; at least, not the way
you're hoping.  It claims that there is no 123 in the
string.	Here's a clearer picture of why it that pattern
matches, contrary to popular expectations:

	   $x = 'ABC123' ;
	   $y = 'ABC445' ;

	   print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
	   print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;

	   print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
	   print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;

This prints

	   2: got ABC
	   3: got AB
	   4: got ABC

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 quanti­
fier (`\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 non-dig­
its, you have something that's not 123?"	 If the pattern
matcher had let `\D*' expand to "ABC", this would have
caused the whole pattern to fail.

The search engine will initially match `\D*' with "ABC".
Then it will try to match `(?!123' with "123", which
fails.  But because a quantifier (`\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, really wants to succeed, so it uses
the standard pattern back-off-and-retry and lets `\D*'
expand to just "AB" this time.  Now there's indeed some­
thing following "AB" that is not "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 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)/ ;
	   print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;

	   6: got ABC

In other words, the two zero-width assertions next to each
other work as though they're ANDed together, just as you'd
use any built-in assertions:  `/^$/' matches only if
you're at the beginning of the line AND the end of the
line simultaneously.  The deeper underlying truth is that
juxtaposition in regular expressions always means AND,
except when you write an explicit OR using the vertical
bar.  `/ab/' means match "a" AND (then) match "b",
although the attempted matches are made at different posi­
tions because "a" is not a zero-width assertion, but a
one-width assertion.

Be aware: particularly complicated regular expressions can
take exponential time to solve because of the immense num­
ber of possible ways they can use backtracking to try
match.  For example, without internal optimizations done
by the regular expression engine, this will take a
painfully long time to run:

	   'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/

And if you used `*''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 what is
known as an "independent group", which does not backtrack
(see the section on "`(?>pattern)'").  Note also that
zero-length look-ahead/look-behind assertions will not
backtrack to make the tail match, since they are in "logi­
cal" context: only whether they match is considered rele­
vant.  For an example where side-effects of look-ahead
might have influenced the following match, see the section
on "`(?>pattern)'".

Version 8 Regular Expressions

In case you're not familiar with the "regular" Version 8
regex routines, here are the pattern-matching rules not
described above.

Any single character matches itself, unless it is a
metacharacter with a special meaning described here or
above.  You can cause characters that normally function as
metacharacters to be interpreted literally by prefixing
them with a "\" (e.g., "\." matches a ".", not any charac­
ter; "\\" matches a "\").  A series of characters matches
that series of characters in the target string, so the
pattern `blurfl' would match "blurfl" in the target
string.

You can specify a character class, by enclosing a list of
characters in `[]', 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 specifies a range, so
that `a-z' represents all characters between "a" and "z",
inclusive.  If you want either "-" or "]" itself to be a
member of a class, put it at the start of the list (possi­
bly after a "^"), or escape it with a backslash.	 "-" is
also taken literally when it is at the end of the list,
just before the closing "]".  (The following all specify
the same class of three characters: `[-az]', `[az-]', and
`[a\-z]'.  All are different from `[a-z]', which specifies
a class containing twenty-six characters.)  Also, if you
try to use the character classes `\w', `\W', `\s', `\S',
`\d', or `\D' as endpoints of a range, that's not a range,
the "-" is understood literally.

Note also that the whole range idea is rather unportable
between character sets--and even within character sets
they may cause results you probably didn't expect.  A
sound principle is to use only ranges that begin from and
end at either alphabets of equal case ([a-e], [A-E]), or
digits ([0-9]).	Anything else is unsafe.  If in doubt,
spell out the character sets in full.

Characters may be specified using a metacharacter syntax
much like that used in C: "\n" matches a newline, "\t" a
tab, "\r" a carriage return, "\f" a form feed, etc.  More
generally, \nnn, where nnn is a string of octal digits,
matches the character whose ASCII value is nnn.	Simi­
larly, \xnn, where nn are hexadecimal digits, matches the
character whose ASCII value is nn. The expression \cx
matches the ASCII character control-x.  Finally, the "."
metacharacter matches any character except "\n" (unless
you use `/s').

You can specify a series of alternatives for a pattern
using "|" to separate them, so that `fee|fie|foe' will
match any of "fee", "fie", or "foe" in the target string
(as would `f(e|i|o)e').	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.  That's why it's common prac­
tice to include alternatives in parentheses: to minimize
confusion about where they start and end.

Alternatives are tried from left to right, so the first
alternative found for which the entire expression matches,
is the one that is chosen. This means that alternatives
are not necessarily greedy. For example: when matching
`foo|foot' against "barefoot", only the "foo" part will
match, as that is the first alternative tried, and it suc­
cessfully matches the target string. (This might not seem
important, but it is important when you are capturing
matched text using parentheses.)

Also remember that "|" is interpreted as a literal within
square brackets, so if you write `[fee|fie|foe]' you're
really only matching `[feio|]'.

Within a pattern, you may designate subpatterns for later
reference by enclosing them in parentheses, and you may
refer back to the nth subpattern later in the pattern
using the metacharacter \n.  Subpatterns are numbered
based on the left to right order of their opening paren­
thesis.	A backreference matches whatever actually matched
the subpattern in the string being examined, not the rules
for that subpattern.  Therefore, `(0|0x)\d*\s\1\d*' will
match "0x1234 0x4321", but not "0x1234 01234", because
subpattern 1 matched "0x", even though the rule `0|0x'
could potentially match the leading 0 in the second num­
ber.

Warning on \1 vs $1

Some people get too used to writing things like:

	   $pattern =~ s/(\W)/\\\1/g;

This is grandfathered for the RHS of a substitute to avoid
shocking the sed addicts, but it's a dirty habit to get
into.  That's because in PerlThink, the righthand side of
a `s///' is a double-quoted string.  `\1' in the usual
double-quoted string means a control-A.	The customary
Unix meaning of `\1' is kludged in for `s///'.  However,
if you get into the habit of doing that, you get yourself
into trouble if you then add an `/e' modifier.

	   s/(\d+)/ \1 + 1 /eg;	# causes warning under -w

Or if you try to do

	   s/(\d+)/\1000/;

You can't disambiguate that by saying `\{1}000', whereas
you can fix it with `${1}000'.  The operation of interpo­
lation should not be confused with the operation of match­
ing a backreference.  Certainly they mean two different
things on the left side of the `s///'.

Repeated patterns matching zero-length substring

Regular expressions provide a terse and powerful program­
ming language.  As with most other power tools, power
comes together with the ability to wreak havoc.

A common abuse of this power stems from the ability to
make infinite loops using regular expressions, with some­
thing as innocuous as:

	   'foo' =~ m{ ( o? )* }x;

The `o?' can match at the beginning of `'foo'', and since
the position in the string is not moved by the match, `o?'
would match again and again because of the `*' modifier.
Another common way to create a similar cycle is with the
looping modifier `//g':

	   @matches = ( 'foo' =~ m{ o? }xg );

or

	   print "match: <$&>\n" while '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 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// /

Thus Perl allows such constructs, by forcefully breaking
the infinite loop.  The rules for this are different for
lower-level loops given by the greedy modifiers `*+{}',
and for higher-level ones like the `/g' modifier or
split() operator.

The lower-level loops are interrupted (that is, the loop
is broken) when Perl detects that a repeated expression
matched a zero-length substring.	  Thus

	  m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

is made equivalent to

	  m{   (?: NON_ZERO_LENGTH )*
	     |
	(?: ZERO_LENGTH )?
	   }x;

The higher level-loops preserve an additional state
between iterations: whether the last match was zero-
length.	To break the loop, the following match after a
zero-length match is prohibited to have a length of zero.
This prohibition interacts with backtracking (see the sec­
tion on "Backtracking"), and so the second best match is
chosen if the best match is of zero length.

For example:

	   $_ = 'bar';
	   s/\w??/<$&>/g;

results in `"<'<b><><a><><r><>">.  At each position of the
string the best match given by non-greedy `??' is the
zero-length match, and the second best match is what is
matched by `\w'.	 Thus zero-length matches alternate with
one-character-long matches.

Similarly, for repeated `m/()/g' the second-best match is
the match at the position one notch further in the string.

The additional state of being matched with zero-length is
associated with the matched string, and is reset by each
assignment to pos().  Zero-length matches at the end of
the previous match are ignored during `split'.

Combining pieces together

Each of the elementary pieces of regular expressions which
were described before (such as `ab' or `\Z') could match
at most one substring at the given position of the input
string.	However, in a typical regular expression these
elementary pieces are combined into more complicated pat­
terns using combining operators `ST', `S|T', `S*' etc (in
these examples `S' and `T' are regular subexpressions).

Such combinations can include alternatives, leading to a
problem of choice: if we match a regular expression `a|ab'
against `"abc"', will it match substring `"a"' or `"ab"'?
One way to describe which substring is actually matched is
the concept of backtracking (see the section on "Back­
tracking").  However, this description is too low-level
and makes you think in terms of a particular implementa­
tion.

Another description starts with notions of "bet­
ter"/"worse".  All the substrings which may be matched by
the given regular expression can be sorted from the "best"
match to the "worst" match, and it is the "best" match
which is chosen.	 This substitutes the question of "what
is chosen?"  by the question of "which matches are better,
and which are worse?".

Again, for elementary pieces there is no such question,
since at most one match at a given position is possible.
This section describes the notion of better/worse for com­
bining operators.  In the description below `S' and `T'
are regular subexpressions.

`ST'
	   Consider two possible matches, `AB' and `A'B'', `A'
	   and `A'' are substrings which can be matched by `S',
	   `B' and `B'' are substrings which can be matched by
	   `T'.

	   If `A' is better match for `S' than `A'', `AB' is a
	   better match than `A'B''.

	   If `A' and `A'' coincide: `AB' is a better match than
	   `AB'' if `B' is better match for `T' than `B''.

`S|T'
	   When `S' can match, it is a better match than when
	   only `T' can match.

	   Ordering of two matches for `S' is the same as for
	   `S'.	 Similar for two matches for `T'.

`S{REPEAT_COUNT}'
	   Matches as `SSS...S' (repeated as many times as neces­
	   sary).

`S{min,max}'
	   Matches as `S{max}|S{max-1}|...|S{min+1}|S{min}'.

`S{min,max}?'
	   Matches as `S{min}|S{min+1}|...|S{max-1}|S{max}'.

`S?', `S*', `S+'
	   Same as `S{0,1}', `S{0,BIG_NUMBER}', `S{1,BIG_NUMBER}'
	   respectively.

`S??', `S*?', `S+?'
	   Same as `S{0,1}?', `S{0,BIG_NUMBER}?',
	   `S{1,BIG_NUMBER}?' respectively.

`(?>S)'
	   Matches the best match for `S' and only that.

`(?=S)', `(?<=S)'
	   Only the best match for `S' is considered.  (This is
	   important only if `S' has capturing parentheses, and
	   backreferences are used somewhere else in the whole
	   regular expression.)

`(?!S)', `(?<!S)'
	   For this grouping operator there is no need to
	   describe the ordering, since only whether or not `S'
	   can match is important.

`(??{ EXPR })'
	   The ordering is the same as for the regular expression
	   which is the result of EXPR.

`(?(condition)yes-pattern|no-pattern)'
	   Recall that which of `yes-pattern' or `no-pattern'
	   actually matches is already determined.  The ordering
	   of the matches is the same as for the chosen subex­
	   pression.

The above recipes describe the ordering of matches at a
given position.	One more rule is needed to understand how
a match is determined for the whole regular expression: a
match at an earlier position is always better than a match
at a later position.

Creating custom RE engines

Overloaded constants (see the overload manpage) provide a
simple way to extend the functionality of the RE engine.

Suppose that we want to enable a new RE escape-sequence
`\Y|' which matches at boundary between white-space char­
acters and non-whitespace characters.  Note that
`(?=\S)(?<!\S)|(?!\S)(?<=\S)' matches exactly at these
positions, so we want to have each `\Y|' in the place of
the more complicated version.  We can create a module
`customre' to do this:

	   package customre;
	   use overload;

	   sub import {
	     shift;
	     die "No argument to customre::import allowed" if @_;
	     overload::constant 'qr' => \&convert;
	   }

	   sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}

	   my %rules = ( '\\' => '\\',
			 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
	   sub convert {
	     my $re = shift;
	     $re =~ s{
		\\ ( \\ | Y . )
		     }
		     { $rules{$1} or invalid($re,$1) }sgex;
	     return $re;
	   }

Now `use customre' enables the new escape in constant reg­
ular expressions, i.e., those without any runtime variable
interpolations.	As documented in the overload manpage,
this conversion will work only over literal parts of regu­
lar expressions.	 For `\Y|$re\Y|' the variable part of
this regular expression needs to be converted explicitly
(but only if the special meaning of `\Y|' should be
enabled inside $re):

	   use customre;
	   $re = <>;
	   chomp $re;
	   $re = customre::convert $re;
	   /\Y|$re\Y|/;


BUGS
This document varies from difficult to understand to com­
pletely and utterly opaque.  The wandering prose riddled
with jargon is hard to fathom in several places.

This document needs a rewrite that separates the tutorial
content from the reference content.

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