perlre.pod

MSYS在windows下模拟了一个类unix的终端

    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 C<(.+)+> is doing, and C<(.+)+> is similar
to a subpattern of the above pattern.  Consider how the pattern
above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several
seconds, but that each extra letter doubles this time.  This
exponential performance will make it appear that your program has
hung.  However, a tiny change to this pattern

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

which uses C<< (?>...) >> 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 C<a>s.  Be aware,
however, that this pattern currently triggers a warning message under
the C<use warnings> pragma or B<-w> switch saying it
C<"matches the null string many times">):

On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable
effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>.
This was only 4 times slower on a string with 1000000 C<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 C<()*> looks like
the correct solution.  Suppose we parse text with comments being delimited
by C<#> followed by some optional (horizontal) whitespace.  Contrary to
its appearance, C<#[ \t]*> I<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:

    (?>#[ \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 specification of comments.

=item C<(?(condition)yes-pattern|no-pattern)>

=item C<(?(condition)yes-pattern)>

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

Conditional expression.  C<(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.

=back

=head2 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 L<Combining pieces together>.

A fundamental feature of regular expression matching involves the
notion called I<backtracking>, which is currently used (when needed)
by all regular expression quantifiers, namely C<*>, C<*?>, C<+>,
C<+?>, C<{n,m}>, and C<{n,m}?>.  Backtracking is often optimized
internally, but the general principle outlined here is valid.

For a regular expression to match, the I<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 expression (C<\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:

    $_ =  "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 C<.*> was greedy, so you get everything between the
I<first> "foo" and the I<last> "bar".  Here it's more effective
to use minimal matching to make sure you get the text between a "foo"
and the first "bar" thereafter.

    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 C<.*> was greedy and gobbled up the
whole string. As C<\d*> can match on an empty string the complete
regular expression matched successfully.

    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 quantifier (C<\D*>) and so can use
backtracking, whereas test 1 will not.  What's happening is
that you've asked "Is it true that at the start of $x, following 0 or more
non-digits, you have something that's not 123?"  If the pattern matcher had
let C<\D*> expand to "ABC", this would have caused the whole pattern to
fail.

The search engine will initially match C<\D*> with "ABC".  Then it will
try to match C<(?!123> with "123", which fails.  But because
a quantifier (C<\D*>) has been used in the regular expression, the
search engine can backtrack and retry the match differently
in the hope of matching the complete regular expression.

The pattern really, I<really> wants to succeed, so it uses the
standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
time.  Now there's indeed something following "AB" that is not
"123".  It's "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:  C</^$/>
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.  C</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.

B<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 C<*>'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 C<{0,5}> instead of C<*>
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 L<C<< (?>pattern) >>>).  Note also that
zero-length look-ahead/look-behind assertions will not backtrack to make
the tail match, since they are in "logical" context: only 
whether they match is considered relevant.  For an example
where side-effects of look-ahead I<might> have influenced the
following match, see L<C<< (?>pattern) >>>.

=head2 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 I<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 C<blurfl>
would match "blurfl" in the target string.

You can specify a character class, by enclosing a list of characters
in C<[]>, which will match any one character from the list.  If the
first character after the "[" is "^", the class matches any character not
in the list.  Within a list, the "-" character specifies a
range, so that C<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: C<[-az]>,
C<[az-]>, and C<[a\-z]>.  All are different from C<[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 C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\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