perlretut.pod

MSYS在windows下模拟了一个类unix的终端

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

The minimal string that will allow both the start of the string C<^>
and the alternation to match is C<Th>, with the alternation C<e|r>
matching C<e>.  The second quantifier C<.*> is free to gobble up the
rest of the string.

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

The first string position that this regexp can match is at the first
C<'m'> in C<programming>. At this position, the minimal C<m{1,2}?>
matches just one C<'m'>.  Although the second quantifier C<.*?> would
prefer to match no characters, it is constrained by the end-of-string
anchor C<$> to match the rest of the string.

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

In this regexp, you might expect the first minimal quantifier C<.*?>
to match the empty string, because it is not constrained by a C<^>
anchor to match the beginning of the word.  Principle 0 applies here,
however.  Because it is possible for the whole regexp to match at the
start of the string, it I<will> match at the start of the string.  Thus
the first quantifier has to match everything up to the first C<m>.  The
second minimal quantifier matches just one C<m> and the third
quantifier matches the rest of the string.

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

Just as in the previous regexp, the first quantifier C<.??> can match
earliest at position C<'a'>, so it does.  The second quantifier is
greedy, so it matches C<mm>, and the third matches the rest of the
string.

We can modify principle 3 above to take into account non-greedy
quantifiers:

=over 4

=item *

Principle 3: If there are two or more elements in a regexp, the
leftmost greedy (non-greedy) quantifier, if any, will match as much
(little) of the string as possible while still allowing the whole
regexp to match.  The next leftmost greedy (non-greedy) quantifier, if
any, will try to match as much (little) 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

Just like alternation, quantifiers are also susceptible to
backtracking.  Here is a step-by-step analysis of the example

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

=over 4

=item 0

Start with the first letter in the string 't'.

=item 1

The first quantifier '.*' starts out by matching the whole
string 'the cat in the hat'.

=item 2

'a' in the regexp element 'at' doesn't match the end of the
string.  Backtrack one character.

=item 3

'a' in the regexp element 'at' still doesn't match the last
letter of the string 't', so backtrack one more character.

=item 4

Now we can match the 'a' and the 't'.

=item 5

Move on to the third element '.*'.  Since we are at the end of
the string and '.*' can match 0 times, assign it the empty string.

=item 6

We are done!

=back

Most of the time, all this moving forward and backtracking happens
quickly and searching is fast.   There are some pathological regexps,
however, whose execution time exponentially grows with the size of the
string.  A typical structure that blows up in your face is of the form

    /(a|b+)*/;

The problem is the nested indeterminate quantifiers.  There are many
different ways of partitioning a string of length n between the C<+>
and C<*>: one repetition with C<b+> of length n, two repetitions with
the first C<b+> length k and the second with length n-k, m repetitions
whose bits add up to length n, etc.  In fact there are an exponential
number of ways to partition a string as a function of length.  A
regexp may get lucky and match early in the process, but if there is
no match, perl will try I<every> possibility before giving up.  So be
careful with nested C<*>'s, C<{n,m}>'s, and C<+>'s.  The book
I<Mastering regular expressions> by Jeffrey Friedl gives a wonderful
discussion of this and other efficiency issues.

=head2 Building a regexp

At this point, we have all the basic regexp concepts covered, so let's
give a more involved example of a regular expression.  We will build a
regexp that matches numbers.

The first task in building a regexp is to decide what we want to match
and what we want to exclude.  In our case, we want to match both
integers and floating point numbers and we want to reject any string
that isn't a number.

The next task is to break the problem down into smaller problems that
are easily converted into a regexp.

The simplest case is integers.  These consist of a sequence of digits,
with an optional sign in front.  The digits we can represent with
C<\d+> and the sign can be matched with C<[+-]>.  Thus the integer
regexp is

    /[+-]?\d+/;  # matches integers

A floating point number potentially has a sign, an integral part, a
decimal point, a fractional part, and an exponent.  One or more of these
parts is optional, so we need to check out the different
possibilities.  Floating point numbers which are in proper form include
123., 0.345, .34, -1e6, and 25.4E-72.  As with integers, the sign out
front is completely optional and can be matched by C<[+-]?>.  We can
see that if there is no exponent, floating point numbers must have a
decimal point, otherwise they are integers.  We might be tempted to
model these with C<\d*\.\d*>, but this would also match just a single
decimal point, which is not a number.  So the three cases of floating
point number sans exponent are

   /[+-]?\d+\./;  # 1., 321., etc.
   /[+-]?\.\d+/;  # .1, .234, etc.
   /[+-]?\d+\.\d+/;  # 1.0, 30.56, etc.

These can be combined into a single regexp with a three-way alternation:

   /[+-]?(\d+\.\d+|\d+\.|\.\d+)/;  # floating point, no exponent

In this alternation, it is important to put C<'\d+\.\d+'> before
C<'\d+\.'>.  If C<'\d+\.'> were first, the regexp would happily match that
and ignore the fractional part of the number.

Now consider floating point numbers with exponents.  The key
observation here is that I<both> integers and numbers with decimal
points are allowed in front of an exponent.  Then exponents, like the
overall sign, are independent of whether we are matching numbers with
or without decimal points, and can be 'decoupled' from the
mantissa.  The overall form of the regexp now becomes clear:

    /^(optional sign)(integer | f.p. mantissa)(optional exponent)$/;

The exponent is an C<e> or C<E>, followed by an integer.  So the
exponent regexp is

   /[eE][+-]?\d+/;  # exponent

Putting all the parts together, we get a regexp that matches numbers:

   /^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/;  # Ta da!

Long regexps like this may impress your friends, but can be hard to
decipher.  In complex situations like this, the C<//x> modifier for a
match is invaluable.  It allows one to put nearly arbitrary whitespace
and comments into a regexp without affecting their meaning.  Using it,
we can rewrite our 'extended' regexp in the more pleasing form

   /^
      [+-]?         # first, match an optional sign
      (             # then match integers or f.p. mantissas:
          \d+\.\d+  # mantissa of the form a.b
         |\d+\.     # mantissa of the form a.
         |\.\d+     # mantissa of the form .b
         |\d+       # integer of the form a
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;

If whitespace is mostly irrelevant, how does one include space
characters in an extended regexp? The answer is to backslash it
S<C<'\ '> > or put it in a character class S<C<[ ]> >.  The same thing
goes for pound signs, use C<\#> or C<[#]>.  For instance, Perl allows
a space between the sign and the mantissa/integer, and we could add
this to our regexp as follows:

   /^
      [+-]?\ *      # first, match an optional sign *and space*
      (             # then match integers or f.p. mantissas:
          \d+\.\d+  # mantissa of the form a.b
         |\d+\.     # mantissa of the form a.
         |\.\d+     # mantissa of the form .b
         |\d+       # integer of the form a
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;

In this form, it is easier to see a way to simplify the
alternation.  Alternatives 1, 2, and 4 all start with C<\d+>, so it
could be factored out:

   /^
      [+-]?\ *      # first, match an optional sign
      (             # then match integers or f.p. mantissas:
          \d+       # start out with a ...
          (
              \.\d* # mantissa of the form a.b or a.
          )?        # ? takes care of integers of the form a
         |\.\d+     # mantissa of the form .b
      )
      ([eE][+-]?\d+)?  # finally, optionally match an exponent
   $/x;

or written in the compact form,

    /^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/;

This is our final regexp.  To recap, we built a regexp by

=over 4

=item *

specifying the task in detail,

=item *

breaking down the problem into smaller parts,

=item *

translating the small parts into regexps,

=item *

combining the regexps,

=item *

and optimizing the final combined regexp.

=back

These are also the typical steps involved in writing a computer
program.  This makes perfect sense, because regular expressions are
essentially programs written a little computer language that specifies
patterns.

=head2 Using regular expressions in Perl

The last topic of Part 1 briefly covers how regexps are used in Perl
programs.  Where do they fit into Perl syntax?

We have already introduced the matching operator in its default
C</regexp/> and arbitrary delimiter C<m!regexp!> forms.  We have used
the binding operator C<=~> and its negation C<!~> to test for string
matches.  Associated with the matching operator, we have discussed the
single line C<//s>, multi-line C<//m>, case-insensitive C<//i> and
extended C<//x> modifiers.

There are a few more things you might want to know about matching
operators.  First, we pointed out earlier that variables in regexps are
substituted before the regexp is evaluated:

    $pattern = 'Seuss';
    while (<>) {
        print if /$pattern/;
    }

This will print any lines containing the word C<Seuss>.  It is not as
efficient as it could be, however, because perl has to re-evaluate
C<$pattern> each time through the loop.  If C<$pattern> won't be
changing over the lifetime of the script, we can add the C<//o>
modifier, which directs perl to only perform variable substitutions
once:

    #!/usr/bin/perl
    #    Improved simple_grep
    $regexp = shift;
    while (<>) {
        print if /$regexp/o;  # a good deal faster
    }

If you change C<$pattern> after the first substitution happens, perl
will ignore it.  If you don't want any substitutions at all, use the
special delimiter C<m''>:

    $pattern = 'Seuss';
    while (<>) {
        print if m'$pattern';  # matches '$pattern', not 'Seuss'
    }

C<m''> acts like single quotes on a regexp; all other C<m> delimiters
act like double quotes.  If the regexp evaluates to the empty string,
the regexp in the I<last successful match> is used instead.  So we have

    "dog" =~ /d/;  # 'd' matches
    "dogbert =~ //;  # this matches the 'd' regexp used before

The final two modifiers C<//g> and C<//c> concern multiple matches.
The modifier C<//g> stands for global matching and allows the the
matching operator to match within a string as many times as possible.
In scalar context, successive invocations against a string will have
`C<//g> jump from match to match, keeping track of position in the
string as it goes along.  You can get or set the position with the
C<pos()> function.

The use of C<//g> is shown in the following example.  Suppose we have
a string that consists of words separated by spaces.  If we know how