perlre.pod

ARM上的如果你对底层感兴趣

know which variety of success you will achieve.

When using lookahead assertions and negations, this can all get even
tricker.  Imagine you'd like to find a sequence of 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 of course fails.  But because
a quantifier (C<\D*>) has been used in the regular expression, the
search engine can backtrack and retry the match differently
in the hope of matching the complete regular expression.

The pattern really, I<really> wants to succeed, so it uses the
standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this
time.  Now there's indeed something following "AB" that is not
"123".  It's in fact "C123", which suffices.

We can deal with this by using both an assertion and a negation.  We'll
say that the first part in $1 must be followed by a digit, and in fact, it
must also be followed by something that's not "123".  Remember that the
lookaheads 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 builtin 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.

One warning: particularly complicated regular expressions can take
exponential time to solve due to the immense number of possible ways they
can use backtracking to try match.  For example this will take a very long
time to run

    /((a{0,5}){0,5}){0,5}/

And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
it would take literally forever--or until you ran out of stack space.

A powerful tool for optimizing such beasts is "independent" groups,
which do not backtrace (see L<C<(?E<gt>pattern)>>).  Note also that
zero-length lookahead/lookbehind assertions will not backtrace to make
the tail match, since they are in "logical" context: only the fact
whether they match or not is considered relevant.  For an example
where side-effects of a lookahead I<might> have influenced the
following match, see L<C<(?E<gt>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 is used to specify a
range, so that C<a-z> represents all characters between "a" and "z",
inclusive.  If you want "-" itself to be a member of a class, put it
at the start or end of the list, or escape it with a backslash.  (The
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.)

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, \I<nnn>, where I<nnn> is a string
of octal digits, matches the character whose ASCII value is I<nnn>.
Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
character whose ASCII value is I<nn>. The expression \cI<x> matches the
ASCII character control-I<x>.  Finally, the "." metacharacter matches any
character except "\n" (unless you use C</s>).

You can specify a series of alternatives for a pattern using "|" to
separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
or "foe" in the target string (as would C<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.  For this reason, 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 mathing C<foo|foot> 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 C<[fee|fie|foe]> you're really only matching C<[feio|]>.

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

=head2 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
B<sed> addicts, but it's a dirty habit to get into.  That's because in
PerlThink, the righthand side of a C<s///> is a double-quoted string.  C<\1> in
the usual double-quoted string means a control-A.  The customary Unix
meaning of C<\1> is kludged in for C<s///>.  However, if you get into the habit
of doing that, you get yourself into trouble if you then add an C</e>
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 C<\{1}000>, whereas you can fix it with
C<${1}000>.  Basically, the operation of interpolation should not be confused
with the operation of matching a backreference.  Certainly they mean two
different things on the I<left> side of the C<s///>.

=head2 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 innocous as:

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

The C<o?> can match at the beginning of C<'foo'>, and since the position
in the string is not moved by the match, C<o?> would match again and again
due to the C<*> modifier.  Another common way to create a similar cycle
is with the looping modifier C<//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 which
may match zero-length substrings, with a simple example being:

    @chars = split //, $string;		  # // is not magic in split
    ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /

Thus Perl allows the C</()/> construct, which I<forcefully breaks
the infinite loop>.  The rules for this are different for lower-level
loops given by the greedy modifiers C<*+{}>, and for higher-level
ones like the C</g> modifier or split() operator.

The lower-level loops are I<interrupted> when it is detected that a 
repeated expression did match 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 L<"Backtracking">), 
and so the I<second best> match is chosen if the I<best> match is of
zero length.

Say,

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

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

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

The additional state of being I<matched with zero-length> is associated to
the matched string, and is reset by each assignment to pos().

=head2 Creating custom RE engines

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

Suppose that we want to enable a new RE escape-sequence C<\Y|> which
matches at boundary between white-space characters and non-whitespace
characters.  Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly
at these positions, so we want to have each C<\Y|> in the place of the
more complicated version.  We can create a module C<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 C<use customre> enables the new escape in constant regular
expressions, i.e., those without any runtime variable interpolations.
As documented in L<overload>, this conversion will work only over
literal parts of regular expressions.  For C<\Y|$re\Y|> the variable
part of this regular expression needs to be converted explicitly
(but only if the special meaning of C<\Y|> should be enabled inside $re):

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

=head2 SEE ALSO

L<perlop/"Regexp Quote-Like Operators">.

L<perlop/"Gory details of parsing quoted constructs">.

L<perlfunc/pos>.

L<perllocale>.

I<Mastering Regular Expressions> (see L<perlbook>) by Jeffrey Friedl.