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PERLRE
!!!PERLRE
NAME
DESCRIPTION
BUGS
SEE ALSO
----
!!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 ``Regexp Quote-Like
Operators'' in perlop.


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


i


Do case-insensitive pattern matching.


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


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 still allowing ``^'' and
``$'' to match, respectively, just after and just before
newlines within the string.


x


Extend your pattern's legibility by permitting whitespace
and comments.


These are usually written as /x
modifier
(?...) construct. See below.


The /x modifier itself needs a little more
explanation. It tells 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
# 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 #
characters in the pattern (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 careful not to include the pattern delimiter in the
comment--perl has no way of knowing you did not intend to
close the pattern early. See the C-comment deletion code in
perlop.


__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
redistributable reimplementation of the V8 routines.) See
``Version 8 Regular Expressions'' 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 ``$'' character only the end (or before the newline at the end), and Perl does certain optimizations with the assumption that the string contains only one line. Embedded newlines will not be matched by ``^'' or ``$''. You may, however, wish to treat a string as a multi-line buffer, such that the ``^'' will match after any 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 $*, 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 single line--even if it isn't. The /s modifier
also overrides 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 platforms. 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 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
??     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 perllocale. For documentation of N{name}, see charnames.


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


In addition, Perl defines the following:


    w  Match a
A w matches a single alphanumeric character or _, not a whole word. Use w+ to match a string of Perl-identifier 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 perllocale. 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 literally. See utf8 for details about pP, PP, and X.


The POSIX character class syntax


    [[:class:]
is also available. The available classes and their backslash equivalents (if available) are as follows:


    alpha
alnum
ascii
blank               [[1]
cntrl
digit       d
graph
lower
print
punct
space       s      [[2]
upper
word        w      [[3]
xdigit
  [[1] A GNU extension equivalent to C
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 zero, one, any alphabetic character, and the percentage sign.


If the utf8 pragma is used, the following
equivalences to Unicode p{} constructs and equivalent
backslash character classes (if available), will
hold:


    alpha       !IsAlpha
alnum       !IsAlnum
ascii       IsASCII
blank       !IsSpace
cntrl       !IsCntrl
digit       !IsDigit        d
graph       !IsGraph
lower       !IsLower
print       !IsPrint
punct       !IsPunct
space       !IsSpace
!IsSpacePerl    s
upper       !IsUpper
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
usual isalpha(3) interface (except for `word' and
`blank').


The assumedly non-obviously named classes are:


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. All
characters with ''ord()'' less than 32 are most often
classified as control characters (assuming
ASCII , the ISO Latin
character sets, and Unicode).


graph


Any alphanumeric or punctuation (special)
character.


print


Any alphanumeric or punctuation (special) character or
space.


punct


Any punctuation (special) character.


xdigit


Any hexadecimal digit. Though this may feel silly
([[0-9A-Fa-f] 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:


    POSIX       trad. Perl  utf8 Perl
    [[:^digit:]      D      P{!IsDigit}
[[:^space:]      S      P{!IsSpace}
[[:^word:]       W      P{!IsWord}
The POSIX character classes [[.cc.] and [[=cc=] are recognized 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 characters 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 ``$'', except that they won't match multiple times when the /m modifier is used, while ``^'' and ``$'' 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 ``Regexp Quote-Like
Operators'' in perlop. It is also useful when writing
lex-like scanners, when you have several patterns
that you want to match against consequent substrings of your
string, see the previous reference. The actual location
where G will match can also be influenced by using
pos() as an lvalue. See ``pos'' in
perlfunc.


The bracketing construct ( ... ) creates capture
buffers. To refer to the digit'th buffer use
$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
character at number 9 in your coded character set; which
would be the 10th character, a horizontal tab under
ASCII .) 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 (/(.)1/) {                 # find first doubled char
print
    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
``Compound Statements'' in perlsyn.)


__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 contains
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 parentheses 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 \, , ),


    $pattern =~ s/(W)/\$1/g;
(If use locale is set, then this depends on the current locale.) Today it is more common to use the ''quotemeta()'' function or the Q metaquoting escape sequence to disable all metacharacters' special meanings like this:


    /$unquotedQ$quotedE$unquoted/
Beware that if you put literal backslashes (those not inside interpolated variables) between Q and E, double-quotish backslash interpolation may lead to confusing results. If you ''need'' to use literal backslashes within Q...E, consult ``Gory details of parsing quoted constructs'' in perlop.


__Extended Patterns__


Perl also defines a consistent extension syntax for features
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. 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
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 patterns, such as those read
in from a configuration 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 pattern. For example:


    $pattern =
    # more flexible:
    $pattern =
Letters after a - turn those modifiers off. These modifiers are localized inside an enclosing 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 backreferences
as ``()'' does. So


    @fields = split(/b(?:abc)b/)
is like


    @fields = split(/b(abc)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 however 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 preceded 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 having three
characters before it. You could cover that this way:
/(?:(?!foo)...^.{0,2})bar/. Sometimes it's still
easier just to say:


    if (/bar/
For look-behind see below.


(?


A zero-width positive look-behind assertion. For example,
/(? matches a word that follows a tab,
without including the tab in $. Works only for
fixed-width look-behind.


(?


A zero-width negative look-behind assertion. For example
/(? matches any occurrence of ``foo''
that does not follow ``bar''. Works only for fixed-width
look-behind.


(?{ code })


__WARNING__ : This extended regular
expression feature 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 ``Backtracking''),
all changes introduced after localization are
undone, so that


  $_ = 'a' x 8;
m
will set $res = 4. Note that after the match, $cnt returns to the globally introduced value, because the scopes that restrict local operators are unwound.


This assertion may be used as a
(?(condition)yes-patternno-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 localized,
so the old value of $^R is restored if the
assertion is backtracked; compare
``Backtracking''.


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


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


    $re =
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 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 perlsec for details about both these mechanisms.


(??{ code })


__WARNING__ : This extended regular
expression feature is considered highly experimental, and
may be changed or deleted without notice. A simplified
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 expression 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{
(?:
(?


(?


__WARNING__ : This extended regular
expression feature is considered highly experimental, and
may be changed or deleted without notice.


An ``independent'' subexpression, one which matches the
substring that a ''standalone'' pattern would
match if anchored at the given position, and it matches
''nothing other than this substring''. This construct is
useful for optimizations of what would otherwise be
``eternal'' matches, because it will not backtrack (see
``Backtracking''). It may also be useful in places where the
``grab all you can, and do not give anything back'' semantic
is desirable.


For example: ^(? will never match, since
(? (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
``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 (? 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 ordinals 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{
(
(?
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 as. Be aware, however, that this pattern currently triggers a warning message under the use warnings pragma or __-w__ switch saying it ):


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
as.


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
# followed 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 whitespace if the
remainder of the pattern can be made to match that way. The
correct answer is either one of these:


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


    / (?
Which one you pick depends on which of these expressions better reflects the above specification of comments.


(?(condition)yes-patternno-pattern)


(?(condition)yes-pattern)


__WARNING__ : This extended regular
expression feature 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__


NOTE: 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
``Combining pieces together''.


A fundamental feature of regular expression matching
involves the notion called ''backtracking'', which is
currently used (when needed) by all regular expression
quantifiers, 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.'':


    $_ =
When the match runs, the first part of the regular expression (b(foo)) 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 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.''


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


    $_ =
Which perhaps unexpectedly yields:


  got
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
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:


    $_ =
That won't work at all, because .* was greedy and gobbled up the whole string. As d* can match on an empty string the complete regular expression matched successfully.


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


    $_ =
    for $pat (@pats) {
printf
That will print out:


    (.*)(d*)
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


    $_ =
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
    print
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 quantifier (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-digits, 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 something 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
    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 positions because ``a'' is not a zero-width assertion, but a one-width assertion.


__WARNING__ : particularly complicated
regular expressions can take exponential time to solve
because of the immense number 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})*[[c]/
And if you used *'s in the internal groups instead of limiting them to 0 through 5 matches, then it would take forever--or until you ran out of stack space. Moreover, these internal optimizations are not always applicable. For example, if you put {0,5} instead of * on the external group, no current optimization is applicable, and the match takes a long time to finish.


A powerful tool for optimizing such beasts is what is known
as an ``independent group'', which does not backtrack (see
(?
might'' have influenced the following match, see
''(?''''


__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 character;
``\'' 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 (possibly 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, even on
EBCDIC based coded character sets.) 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 coded character
set value is ''nnn''. Similarly, x''nn'', where
''nn'' are hexadecimal digits, matches the character
whose numeric value is ''nn''. The expression c''x''
matches the 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 feefiefoe will match
any of ``fee'', ``fie'', or ``foe'' in the target string (as
would f(eio)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 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
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
foofoot against ``barefoot'', only the ``foo'' part
will match, as that is the first alternative tried, and it
successfully 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 [[feefiefoe] 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 ''n''th subpattern later in the pattern
using the metacharacter \''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, (00x)d*s1d*
will match ``0x1234 0x4321'', but not ``0x1234 01234'',
because subpattern 1 matched ``0x'', even though the rule
00x could potentially match the leading 0 in the
second number.


__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 interpolation should not be confused with the operation of matching a backreference. Certainly they mean two different things on the ''left'' side of the s///.


__Repeated patterns matching zero-length
substring__


__WARNING__ : Difficult material (and
prose) ahead. This section needs a rewrite.


Regular expressions provide a terse and powerful programming
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 something 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
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 ``Backtracking''), and so the ''second best'' match is chosen if the ''best'' match is of zero length.


For example:


    $_ = 'bar';
s/w??/
results in . 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
patterns using combining operators ST, ST,
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
aab against , will it match
substring  or ?
One way to describe which substring is actually matched is
the concept of backtracking (see ``Backtracking''). However,
this description is too low-level and makes you think in
terms of a particular implementation.


Another description starts with notions of
``better''/``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
combining 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'.


ST


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
necessary).


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.


(?


Matches the best match for S and only
that.


(?=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), (?


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-patternno-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
subexpression.


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 overload) 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 characters and non-whitespace characters. Note
that (?=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
    sub invalid { die
    my %rules = ( '\' =
Now use customre enables the new escape in constant regular expressions, i.e., those without any runtime variable interpolations. As documented in overload, this conversion will work only over literal parts of regular expressions. For Y$reY 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 =
!!BUGS


This document varies from difficult to understand to
completely 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.
!!SEE ALSO


``Regexp Quote-Like Operators'' in perlop.


``Gory details of parsing quoted constructs'' in
perlop.


perlfaq6.


``pos'' in perlfunc.


perllocale.


perlebcdic.


''Mastering Regular Expressions'' by Jeffrey Friedl,
published by O'Reilly and Associates.
----
4 pages link to perlre(1):
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Last edited on Tuesday, June 4, 2002 12:22:38 am by "perry"
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