perlretut.pod

MSYS在windows下模拟了一个类unix的终端

possible character positions have been exhausted does perl give give
up and declare S<C<$string =~ /(abd|abc)(df|d|de)/;> > to be false.

Even with all this work, regexp matching happens remarkably fast.  To
speed things up, during compilation stage, perl compiles the regexp
into a compact sequence of opcodes that can often fit inside a
processor cache.  When the code is executed, these opcodes can then run
at full throttle and search very quickly.

=head2 Extracting matches

The grouping metacharacters C<()> also serve another completely
different function: they allow the extraction of the parts of a string
that matched.  This is very useful to find out what matched and for
text processing in general.  For each grouping, the part that matched
inside goes into the special variables C<$1>, C<$2>, etc.  They can be
used just as ordinary variables:

    # extract hours, minutes, seconds
    $time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
    $hours = $1;
    $minutes = $2;
    $seconds = $3;

Now, we know that in scalar context,
S<C<$time =~ /(\d\d):(\d\d):(\d\d)/> > returns a true or false
value.  In list context, however, it returns the list of matched values
C<($1,$2,$3)>.  So we could write the code more compactly as

    # extract hours, minutes, seconds
    ($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

If the groupings in a regexp are nested, C<$1> gets the group with the
leftmost opening parenthesis, C<$2> the next opening parenthesis,
etc.  For example, here is a complex regexp and the matching variables
indicated below it:

    /(ab(cd|ef)((gi)|j))/;
     1  2      34

so that if the regexp matched, e.g., C<$2> would contain 'cd' or 'ef'.
For convenience, perl sets C<$+> to the highest numbered C<$1>, C<$2>,
... that got assigned.

Closely associated with the matching variables C<$1>, C<$2>, ... are
the B<backreferences> C<\1>, C<\2>, ... .  Backreferences are simply
matching variables that can be used I<inside> a regexp.  This is a
really nice feature - what matches later in a regexp can depend on
what matched earlier in the regexp.  Suppose we wanted to look
for doubled words in text, like 'the the'.  The following regexp finds
all 3-letter doubles with a space in between:

    /(\w\w\w)\s\1/;

The grouping assigns a value to \1, so that the same 3 letter sequence
is used for both parts.  Here are some words with repeated parts:

    % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\1$' /usr/dict/words
    beriberi
    booboo
    coco
    mama
    murmur
    papa

The regexp has a single grouping which considers 4-letter
combinations, then 3-letter combinations, etc.  and uses C<\1> to look for
a repeat.  Although C<$1> and C<\1> represent the same thing, care should be
taken to use matched variables C<$1>, C<$2>, ... only outside a regexp
and backreferences C<\1>, C<\2>, ... only inside a regexp; not doing
so may lead to surprising and/or undefined results.

In addition to what was matched, Perl 5.6.0 also provides the
positions of what was matched with the C<@-> and C<@+>
arrays. C<$-[0]> is the position of the start of the entire match and
C<$+[0]> is the position of the end. Similarly, C<$-[n]> is the
position of the start of the C<$n> match and C<$+[n]> is the position
of the end. If C<$n> is undefined, so are C<$-[n]> and C<$+[n]>. Then
this code

    $x = "Mmm...donut, thought Homer";
    $x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches
    foreach $expr (1..$#-) {
        print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n";
    }

prints

    Match 1: 'Mmm' at position (0,3)
    Match 2: 'donut' at position (6,11)

Even if there are no groupings in a regexp, it is still possible to
find out what exactly matched in a string.  If you use them, perl
will set C<$`> to the part of the string before the match, will set C<$&>
to the part of the string that matched, and will set C<$'> to the part
of the string after the match.  An example:

    $x = "the cat caught the mouse";
    $x =~ /cat/;  # $` = 'the ', $& = 'cat', $' = ' caught the mouse'
    $x =~ /the/;  # $` = '', $& = 'the', $' = ' cat caught the mouse'

In the second match, S<C<$` = ''> > because the regexp matched at the
first character position in the string and stopped, it never saw the
second 'the'.  It is important to note that using C<$`> and C<$'>
slows down regexp matching quite a bit, and C< $& > slows it down to a
lesser extent, because if they are used in one regexp in a program,
they are generated for <all> regexps in the program.  So if raw
performance is a goal of your application, they should be avoided.
If you need them, use C<@-> and C<@+> instead:

    $` is the same as substr( $x, 0, $-[0] )
    $& is the same as substr( $x, $-[0], $+[0]-$-[0] )
    $' is the same as substr( $x, $+[0] )

=head2 Matching repetitions

The examples in the previous section display an annoying weakness.  We
were only matching 3-letter words, or syllables of 4 letters or
less.  We'd like to be able to match words or syllables of any length,
without writing out tedious alternatives like
C<\w\w\w\w|\w\w\w|\w\w|\w>.

This is exactly the problem the B<quantifier> metacharacters C<?>,
C<*>, C<+>, and C<{}> were created for.  They allow us to determine the
number of repeats of a portion of a regexp we consider to be a
match.  Quantifiers are put immediately after the character, character
class, or grouping that we want to specify.  They have the following
meanings:

=over 4

=item *

C<a?> = match 'a' 1 or 0 times

=item *

C<a*> = match 'a' 0 or more times, i.e., any number of times

=item *

C<a+> = match 'a' 1 or more times, i.e., at least once

=item *

C<a{n,m}> = match at least C<n> times, but not more than C<m>
times.

=item *

C<a{n,}> = match at least C<n> or more times

=item *

C<a{n}> = match exactly C<n> times

=back

Here are some examples:

    /[a-z]+\s+\d*/;  # match a lowercase word, at least some space, and
                     # any number of digits
    /(\w+)\s+\1/;    # match doubled words of arbitrary length
    /y(es)?/i;       # matches 'y', 'Y', or a case-insensitive 'yes'
    $year =~ /\d{2,4}/;  # make sure year is at least 2 but not more
                         # than 4 digits
    $year =~ /\d{4}|\d{2}/;    # better match; throw out 3 digit dates
    $year =~ /\d{2}(\d{2})?/;  # same thing written differently. However,
                               # this produces $1 and the other does not.

    % simple_grep '^(\w+)\1$' /usr/dict/words   # isn't this easier?
    beriberi
    booboo
    coco
    mama
    murmur
    papa

For all of these quantifiers, perl will try to match as much of the
string as possible, while still allowing the regexp to succeed.  Thus
with C</a?.../>, perl will first try to match the regexp with the C<a>
present; if that fails, perl will try to match the regexp without the
C<a> present.  For the quantifier C<*>, we get the following:

    $x = "the cat in the hat";
    $x =~ /^(.*)(cat)(.*)$/; # matches,
                             # $1 = 'the '
                             # $2 = 'cat'
                             # $3 = ' in the hat'

Which is what we might expect, the match finds the only C<cat> in the
string and locks onto it.  Consider, however, this regexp:

    $x =~ /^(.*)(at)(.*)$/; # matches,
                            # $1 = 'the cat in the h'
                            # $2 = 'at'
                            # $3 = ''   (0 matches)

One might initially guess that perl would find the C<at> in C<cat> and
stop there, but that wouldn't give the longest possible string to the
first quantifier C<.*>.  Instead, the first quantifier C<.*> grabs as
much of the string as possible while still having the regexp match.  In
this example, that means having the C<at> sequence with the final C<at>
in the string.  The other important principle illustrated here is that
when there are two or more elements in a regexp, the I<leftmost>
quantifier, if there is one, gets to grab as much the string as
possible, leaving the rest of the regexp to fight over scraps.  Thus in
our example, the first quantifier C<.*> grabs most of the string, while
the second quantifier C<.*> gets the empty string.   Quantifiers that
grab as much of the string as possible are called B<maximal match> or
B<greedy> quantifiers.

When a regexp can match a string in several different ways, we can use
the principles above to predict which way the regexp will match:

=over 4

=item *

Principle 0: Taken as a whole, any regexp will be matched at the
earliest possible position in the string.

=item *

Principle 1: In an alternation C<a|b|c...>, the leftmost alternative
that allows a match for the whole regexp will be the one used.

=item *

Principle 2: The maximal matching quantifiers C<?>, C<*>, C<+> and
C<{n,m}> will in general match as much of the string as possible while
still allowing the whole regexp to match.

=item *

Principle 3: If there are two or more elements in a regexp, the
leftmost greedy quantifier, if any, will match as much of the string
as possible while still allowing the whole regexp to match.  The next
leftmost greedy quantifier, if any, will try to match as much of the
string remaining available to it as possible, while still allowing the
whole regexp to match.  And so on, until all the regexp elements are
satisfied.

=back

As we have seen above, Principle 0 overrides the others - the regexp
will be matched as early as possible, with the other principles
determining how the regexp matches at that earliest character
position.

Here is an example of these principles in action:

    $x = "The programming republic of Perl";
    $x =~ /^(.+)(e|r)(.*)$/;  # matches,
                              # $1 = 'The programming republic of Pe'
                              # $2 = 'r'
                              # $3 = 'l'

This regexp matches at the earliest string position, C<'T'>.  One
might think that C<e>, being leftmost in the alternation, would be
matched, but C<r> produces the longest string in the first quantifier.

    $x =~ /(m{1,2})(.*)$/;  # matches,
                            # $1 = 'mm'
                            # $2 = 'ing republic of Perl'

Here, The earliest possible match is at the first C<'m'> in
C<programming>. C<m{1,2}> is the first quantifier, so it gets to match
a maximal C<mm>.

    $x =~ /.*(m{1,2})(.*)$/;  # matches,
                              # $1 = 'm'
                              # $2 = 'ing republic of Perl'

Here, the regexp matches at the start of the string. The first
quantifier C<.*> grabs as much as possible, leaving just a single
C<'m'> for the second quantifier C<m{1,2}>.

    $x =~ /(.?)(m{1,2})(.*)$/;  # matches,
                                # $1 = 'a'
                                # $2 = 'mm'
                                # $3 = 'ing republic of Perl'

Here, C<.?> eats its maximal one character at the earliest possible
position in the string, C<'a'> in C<programming>, leaving C<m{1,2}>
the opportunity to match both C<m>'s. Finally,

    "aXXXb" =~ /(X*)/; # matches with $1 = ''

because it can match zero copies of C<'X'> at the beginning of the
string.  If you definitely want to match at least one C<'X'>, use
C<X+>, not C<X*>.

Sometimes greed is not good.  At times, we would like quantifiers to
match a I<minimal> piece of string, rather than a maximal piece.  For
this purpose, Larry Wall created the S<B<minimal match> > or
B<non-greedy> quantifiers C<??>,C<*?>, C<+?>, and C<{}?>.  These are
the usual quantifiers with a C<?> appended to them.  They have the
following meanings:

=over 4

=item *

C<a??> = match 'a' 0 or 1 times. Try 0 first, then 1.

=item *

C<a*?> = match 'a' 0 or more times, i.e., any number of times,
but as few times as possible

=item *

C<a+?> = match 'a' 1 or more times, i.e., at least once, but
as few times as possible

=item *

C<a{n,m}?> = match at least C<n> times, not more than C<m>
times, as few times as possible

=item *

C<a{n,}?> = match at least C<n> times, but as few times as
possible

=item *

C<a{n}?> = match exactly C<n> times.  Because we match exactly
C<n> times, C<a{n}?> is equivalent to C<a{n}> and is just there for
notational consistency.

=back

Let's look at the example above, but with minimal quantifiers: