source: trunk/essentials/dev-lang/perl/pod/perlretut.pod@ 3298

=head1 NAME

perlretut - Perl regular expressions tutorial

=head1 DESCRIPTION

This page provides a basic tutorial on understanding, creating and
using regular expressions in Perl.  It serves as a complement to the
reference page on regular expressions L<perlre>.  Regular expressions
are an integral part of the C<m//>, C<s///>, C<qr//> and C<split>
operators and so this tutorial also overlaps with
L<perlop/"Regexp Quote-Like Operators"> and L<perlfunc/split>.

Perl is widely renowned for excellence in text processing, and regular
expressions are one of the big factors behind this fame.  Perl regular
expressions display an efficiency and flexibility unknown in most
other computer languages.  Mastering even the basics of regular
expressions will allow you to manipulate text with surprising ease.

What is a regular expression?  A regular expression is simply a string
that describes a pattern.  Patterns are in common use these days;
examples are the patterns typed into a search engine to find web pages
and the patterns used to list files in a directory, e.g., C<ls *.txt>
or C<dir *.*>.  In Perl, the patterns described by regular expressions
are used to search strings, extract desired parts of strings, and to
do search and replace operations.

Regular expressions have the undeserved reputation of being abstract
and difficult to understand.  Regular expressions are constructed using
simple concepts like conditionals and loops and are no more difficult
to understand than the corresponding C<if> conditionals and C<while>
loops in the Perl language itself.  In fact, the main challenge in
learning regular expressions is just getting used to the terse
notation used to express these concepts.

This tutorial flattens the learning curve by discussing regular
expression concepts, along with their notation, one at a time and with
many examples.  The first part of the tutorial will progress from the
simplest word searches to the basic regular expression concepts.  If
you master the first part, you will have all the tools needed to solve
about 98% of your needs.  The second part of the tutorial is for those
comfortable with the basics and hungry for more power tools.  It
discusses the more advanced regular expression operators and
introduces the latest cutting edge innovations in 5.6.0.

A note: to save time, 'regular expression' is often abbreviated as
regexp or regex.  Regexp is a more natural abbreviation than regex, but
is harder to pronounce.  The Perl pod documentation is evenly split on
regexp vs regex; in Perl, there is more than one way to abbreviate it.
We'll use regexp in this tutorial.

=head1 Part 1: The basics

=head2 Simple word matching

The simplest regexp is simply a word, or more generally, a string of
characters.  A regexp consisting of a word matches any string that
contains that word:

    "Hello World" =~ /World/;  # matches

What is this perl statement all about? C<"Hello World"> is a simple
double quoted string.  C<World> is the regular expression and the
C<//> enclosing C</World/> tells perl to search a string for a match.
The operator C<=~> associates the string with the regexp match and
produces a true value if the regexp matched, or false if the regexp
did not match.  In our case, C<World> matches the second word in
C<"Hello World">, so the expression is true.  Expressions like this
are useful in conditionals:

    if ("Hello World" =~ /World/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }

There are useful variations on this theme.  The sense of the match can
be reversed by using C<!~> operator:

    if ("Hello World" !~ /World/) {
        print "It doesn't match\n";
    }
    else {
        print "It matches\n";
    }

The literal string in the regexp can be replaced by a variable:

    $greeting = "World";
    if ("Hello World" =~ /$greeting/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }

If you're matching against the special default variable C<$_>, the
C<$_ =~> part can be omitted:

    $_ = "Hello World";
    if (/World/) {
        print "It matches\n";
    }
    else {
        print "It doesn't match\n";
    }

And finally, the C<//> default delimiters for a match can be changed
to arbitrary delimiters by putting an C<'m'> out front:

    "Hello World" =~ m!World!;   # matches, delimited by '!'
    "Hello World" =~ m{World};   # matches, note the matching '{}'
    "/usr/bin/perl" =~ m"/perl"; # matches after '/usr/bin',
                                 # '/' becomes an ordinary char

C</World/>, C<m!World!>, and C<m{World}> all represent the
same thing.  When, e.g., C<""> is used as a delimiter, the forward
slash C<'/'> becomes an ordinary character and can be used in a regexp
without trouble.

Let's consider how different regexps would match C<"Hello World">:

    "Hello World" =~ /world/;  # doesn't match
    "Hello World" =~ /o W/;    # matches
    "Hello World" =~ /oW/;     # doesn't match
    "Hello World" =~ /World /; # doesn't match

The first regexp C<world> doesn't match because regexps are
case-sensitive.  The second regexp matches because the substring
S<C<'o W'> > occurs in the string S<C<"Hello World"> >.  The space
character ' ' is treated like any other character in a regexp and is
needed to match in this case.  The lack of a space character is the
reason the third regexp C<'oW'> doesn't match.  The fourth regexp
C<'World '> doesn't match because there is a space at the end of the
regexp, but not at the end of the string.  The lesson here is that
regexps must match a part of the string I<exactly> in order for the
statement to be true.

If a regexp matches in more than one place in the string, perl will
always match at the earliest possible point in the string:

    "Hello World" =~ /o/;       # matches 'o' in 'Hello'
    "That hat is red" =~ /hat/; # matches 'hat' in 'That'

With respect to character matching, there are a few more points you
need to know about.   First of all, not all characters can be used 'as
is' in a match.  Some characters, called B<metacharacters>, are reserved
for use in regexp notation.  The metacharacters are

    {}[]()^$.|*+?\

The significance of each of these will be explained
in the rest of the tutorial, but for now, it is important only to know
that a metacharacter can be matched by putting a backslash before it:

    "2+2=4" =~ /2+2/;    # doesn't match, + is a metacharacter
    "2+2=4" =~ /2\+2/;   # matches, \+ is treated like an ordinary +
    "The interval is [0,1)." =~ /[0,1)./     # is a syntax error!
    "The interval is [0,1)." =~ /\[0,1\)\./  # matches
    "/usr/bin/perl" =~ /\/usr\/bin\/perl/;  # matches

In the last regexp, the forward slash C<'/'> is also backslashed,
because it is used to delimit the regexp.  This can lead to LTS
(leaning toothpick syndrome), however, and it is often more readable
to change delimiters.

    "/usr/bin/perl" =~ m!/usr/bin/perl!;    # easier to read

The backslash character C<'\'> is a metacharacter itself and needs to
be backslashed:

    'C:\WIN32' =~ /C:\\WIN/;   # matches

In addition to the metacharacters, there are some ASCII characters
which don't have printable character equivalents and are instead
represented by B<escape sequences>.  Common examples are C<\t> for a
tab, C<\n> for a newline, C<\r> for a carriage return and C<\a> for a
bell.  If your string is better thought of as a sequence of arbitrary
bytes, the octal escape sequence, e.g., C<\033>, or hexadecimal escape
sequence, e.g., C<\x1B> may be a more natural representation for your
bytes.  Here are some examples of escapes:

    "1000\t2000" =~ m(0\t2)   # matches
    "1000\n2000" =~ /0\n20/   # matches
    "1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"
    "cat"        =~ /\143\x61\x74/ # matches, but a weird way to spell cat

If you've been around Perl a while, all this talk of escape sequences
may seem familiar.  Similar escape sequences are used in double-quoted
strings and in fact the regexps in Perl are mostly treated as
double-quoted strings.  This means that variables can be used in
regexps as well.  Just like double-quoted strings, the values of the
variables in the regexp will be substituted in before the regexp is
evaluated for matching purposes.  So we have:

    $foo = 'house';
    'housecat' =~ /$foo/;      # matches
    'cathouse' =~ /cat$foo/;   # matches
    'housecat' =~ /${foo}cat/; # matches

So far, so good.  With the knowledge above you can already perform
searches with just about any literal string regexp you can dream up.
Here is a I<very simple> emulation of the Unix grep program:

    % cat > simple_grep
    #!/usr/bin/perl
    $regexp = shift;
    while (<>) {
        print if /$regexp/;
    }
    ^D

    % chmod +x simple_grep

    % simple_grep abba /usr/dict/words
    Babbage
    cabbage
    cabbages
    sabbath
    Sabbathize
    Sabbathizes
    sabbatical
    scabbard
    scabbards

This program is easy to understand.  C<#!/usr/bin/perl> is the standard
way to invoke a perl program from the shell.
S<C<$regexp = shift;> > saves the first command line argument as the
regexp to be used, leaving the rest of the command line arguments to
be treated as files.  S<C<< while (<>) >> > loops over all the lines in
all the files.  For each line, S<C<print if /$regexp/;> > prints the
line if the regexp matches the line.  In this line, both C<print> and
C</$regexp/> use the default variable C<$_> implicitly.

With all of the regexps above, if the regexp matched anywhere in the
string, it was considered a match.  Sometimes, however, we'd like to
specify I<where> in the string the regexp should try to match.  To do
this, we would use the B<anchor> metacharacters C<^> and C<$>.  The
anchor C<^> means match at the beginning of the string and the anchor
C<$> means match at the end of the string, or before a newline at the
end of the string.  Here is how they are used:

    "housekeeper" =~ /keeper/;    # matches
    "housekeeper" =~ /^keeper/;   # doesn't match
    "housekeeper" =~ /keeper$/;   # matches
    "housekeeper\n" =~ /keeper$/; # matches

The second regexp doesn't match because C<^> constrains C<keeper> to
match only at the beginning of the string, but C<"housekeeper"> has
keeper starting in the middle.  The third regexp does match, since the
C<$> constrains C<keeper> to match only at the end of the string.

When both C<^> and C<$> are used at the same time, the regexp has to
match both the beginning and the end of the string, i.e., the regexp
matches the whole string.  Consider

    "keeper" =~ /^keep$/;      # doesn't match
    "keeper" =~ /^keeper$/;    # matches
    ""       =~ /^$/;          # ^$ matches an empty string

The first regexp doesn't match because the string has more to it than
C<keep>.  Since the second regexp is exactly the string, it
matches.  Using both C<^> and C<$> in a regexp forces the complete
string to match, so it gives you complete control over which strings
match and which don't.  Suppose you are looking for a fellow named
bert, off in a string by himself:

    "dogbert" =~ /bert/;   # matches, but not what you want

    "dilbert" =~ /^bert/;  # doesn't match, but ..
    "bertram" =~ /^bert/;  # matches, so still not good enough

    "bertram" =~ /^bert$/; # doesn't match, good
    "dilbert" =~ /^bert$/; # doesn't match, good
    "bert"    =~ /^bert$/; # matches, perfect

Of course, in the case of a literal string, one could just as easily
use the string equivalence S<C<$string eq 'bert'> > and it would be
more efficient.   The  C<^...$> regexp really becomes useful when we
add in the more powerful regexp tools below.

=head2 Using character classes

Although one can already do quite a lot with the literal string
regexps above, we've only scratched the surface of regular expression
technology.  In this and subsequent sections we will introduce regexp
concepts (and associated metacharacter notations) that will allow a
regexp to not just represent a single character sequence, but a I<whole
class> of them.

One such concept is that of a B<character class>.  A character class
allows a set of possible characters, rather than just a single
character, to match at a particular point in a regexp.  Character
classes are denoted by brackets C<[...]>, with the set of characters
to be possibly matched inside.  Here are some examples:

    /cat/;       # matches 'cat'
    /[bcr]at/;   # matches 'bat, 'cat', or 'rat'
    /item[0123456789]/;  # matches 'item0' or ... or 'item9'
    "abc" =~ /[cab]/;    # matches 'a'

In the last statement, even though C<'c'> is the first character in
the class, C<'a'> matches because the first character position in the
string is the earliest point at which the regexp can match.

    /[yY][eE][sS]/;      # match 'yes' in a case-insensitive way
                         # 'yes', 'Yes', 'YES', etc.

This regexp displays a common task: perform a case-insensitive
match.  Perl provides away of avoiding all those brackets by simply
appending an C<'i'> to the end of the match.  Then C</[yY][eE][sS]/;>
can be rewritten as C</yes/i;>.  The C<'i'> stands for
case-insensitive and is an example of a B<modifier> of the matching
operation.  We will meet other modifiers later in the tutorial.

We saw in the section above that there were ordinary characters, which
represented themselves, and special characters, which needed a
backslash C<\> to represent themselves.  The same is true in a
character class, but the sets of ordinary and special characters
inside a character class are different than those outside a character
class.  The special characters for a character class are C<-]\^$>.  C<]>
is special because it denotes the end of a character class.  C<$> is
special because it denotes a scalar variable.  C<\> is special because
it is used in escape sequences, just like above.  Here is how the
special characters C<]$\> are handled:

   /[\]c]def/; # matches ']def' or 'cdef'
   $x = 'bcr';
   /[$x]at/;   # matches 'bat', 'cat', or 'rat'
   /[\$x]at/;  # matches '$at' or 'xat'
   /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'

The last two are a little tricky.  in C<[\$x]>, the backslash protects
the dollar sign, so the character class has two members C<$> and C<x>.
In C<[\\$x]>, the backslash is protected, so C<$x> is treated as a
variable and substituted in double quote fashion.

The special character C<'-'> acts as a range operator within character
classes, so that a contiguous set of characters can be written as a
range.  With ranges, the unwieldy C<[0123456789]> and C<[abc...xyz]>
become the svelte C<[0-9]> and C<[a-z]>.  Some examples are

    /item[0-9]/;  # matches 'item0' or ... or 'item9'
    /[0-9bx-z]aa/;  # matches '0aa', ..., '9aa',
                    # 'baa', 'xaa', 'yaa', or 'zaa'
    /[0-9a-fA-F]/;  # matches a hexadecimal digit
    /[0-9a-zA-Z_]/; # matches a "word" character,
                    # like those in a perl variable name

If C<'-'> is the first or last character in a character class, it is
treated as an ordinary character; C<[-ab]>, C<[ab-]> and C<[a\-b]> are
all equivalent.

The special character C<^> in the first position of a character class
denotes a B<negated character class>, which matches any character but
those in the brackets.  Both C<[...]> and C<[^...]> must match a
character, or the match fails.  Then

    /[^a]at/;  # doesn't match 'aat' or 'at', but matches
               # all other 'bat', 'cat, '0at', '%at', etc.
    /[^0-9]/;  # matches a non-numeric character
    /[a^]at/;  # matches 'aat' or '^at'; here '^' is ordinary

Now, even C<[0-9]> can be a bother the write multiple times, so in the
interest of saving keystrokes and making regexps more readable, Perl
has several abbreviations for common character classes:

=over 4

=item *

\d is a digit and represents [0-9]

=item *

\s is a whitespace character and represents [\ \t\r\n\f]

=item *

\w is a word character (alphanumeric or _) and represents [0-9a-zA-Z_]

=item *

\D is a negated \d; it represents any character but a digit [^0-9]

=item *

\S is a negated \s; it represents any non-whitespace character [^\s]

=item *

\W is a negated \w; it represents any non-word character [^\w]

=item *

The period '.' matches any character but "\n"

=back

The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside
of character classes.  Here are some in use:

    /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format
    /[\d\s]/;         # matches any digit or whitespace character
    /\w\W\w/;         # matches a word char, followed by a
                      # non-word char, followed by a word char
    /..rt/;           # matches any two chars, followed by 'rt'
    /end\./;          # matches 'end.'
    /end[.]/;         # same thing, matches 'end.'

Because a period is a metacharacter, it needs to be escaped to match
as an ordinary period. Because, for example, C<\d> and C<\w> are sets
of characters, it is incorrect to think of C<[^\d\w]> as C<[\D\W]>; in
fact C<[^\d\w]> is the same as C<[^\w]>, which is the same as
C<[\W]>. Think DeMorgan's laws.

An anchor useful in basic regexps is the S<B<word anchor> >
C<\b>.  This matches a boundary between a word character and a non-word
character C<\w\W> or C<\W\w>:

    $x = "Housecat catenates house and cat";
    $x =~ /cat/;    # matches cat in 'housecat'
    $x =~ /\bcat/;  # matches cat in 'catenates'
    $x =~ /cat\b/;  # matches cat in 'housecat'
    $x =~ /\bcat\b/;  # matches 'cat' at end of string

Note in the last example, the end of the string is considered a word
boundary.

You might wonder why C<'.'> matches everything but C<"\n"> - why not
every character? The reason is that often one is matching against
lines and would like to ignore the newline characters.  For instance,
while the string C<"\n"> represents one line, we would like to think
of as empty.  Then

    ""   =~ /^$/;    # matches
    "\n" =~ /^$/;    # matches, "\n" is ignored

    ""   =~ /./;      # doesn't match; it needs a char
    ""   =~ /^.$/;    # doesn't match; it needs a char
    "\n" =~ /^.$/;    # doesn't match; it needs a char other than "\n"
    "a"  =~ /^.$/;    # matches
    "a\n"  =~ /^.$/;  # matches, ignores the "\n"

This behavior is convenient, because we usually want to ignore
newlines when we count and match characters in a line.  Sometimes,
however, we want to keep track of newlines.  We might even want C<^>
and C<$> to anchor at the beginning and end of lines within the
string, rather than just the beginning and end of the string.  Perl
allows us to choose between ignoring and paying attention to newlines
by using the C<//s> and C<//m> modifiers.  C<//s> and C<//m> stand for
single line and multi-line and they determine whether a string is to
be treated as one continuous string, or as a set of lines.  The two
modifiers affect two aspects of how the regexp is interpreted: 1) how
the C<'.'> character class is defined, and 2) where the anchors C<^>
and C<$> are able to match.  Here are the four possible combinations:

=over 4

=item *

no modifiers (//): Default behavior.  C<'.'> matches any character
except C<"\n">.  C<^> matches only at the beginning of the string and
C<$> matches only at the end or before a newline at the end.

=item *

s modifier (//s): Treat string as a single long line.  C<'.'> matches
any character, even C<"\n">.  C<^> matches only at the beginning of
the string and C<$> matches only at the end or before a newline at the
end.

=item *

m modifier (//m): Treat string as a set of multiple lines.  C<'.'>
matches any character except C<"\n">.  C<^> and C<$> are able to match
at the start or end of I<any> line within the string.

=item *

both s and m modifiers (//sm): Treat string as a single long line, but
detect multiple lines.  C<'.'> matches any character, even
C<"\n">.  C<^> and C<$>, however, are able to match at the start or end
of I<any> line within the string.

=back

Here are examples of C<//s> and C<//m> in action:

    $x = "There once was a girl\nWho programmed in Perl\n";

    $x =~ /^Who/;   # doesn't match, "Who" not at start of string
    $x =~ /^Who/s;  # doesn't match, "Who" not at start of string
    $x =~ /^Who/m;  # matches, "Who" at start of second line
    $x =~ /^Who/sm; # matches, "Who" at start of second line

    $x =~ /girl.Who/;   # doesn't match, "." doesn't match "\n"
    $x =~ /girl.Who/s;  # matches, "." matches "\n"
    $x =~ /girl.Who/m;  # doesn't match, "." doesn't match "\n"
    $x =~ /girl.Who/sm; # matches, "." matches "\n"

Most of the time, the default behavior is what is want, but C<//s> and
C<//m> are occasionally very useful.  If C<//m> is being used, the start
of the string can still be matched with C<\A> and the end of string
can still be matched with the anchors C<\Z> (matches both the end and
the newline before, like C<$>), and C<\z> (matches only the end):

    $x =~ /^Who/m;   # matches, "Who" at start of second line
    $x =~ /\AWho/m;  # doesn't match, "Who" is not at start of string

    $x =~ /girl$/m;  # matches, "girl" at end of first line
    $x =~ /girl\Z/m; # doesn't match, "girl" is not at end of string

    $x =~ /Perl\Z/m; # matches, "Perl" is at newline before end
    $x =~ /Perl\z/m; # doesn't match, "Perl" is not at end of string

We now know how to create choices among classes of characters in a
regexp.  What about choices among words or character strings? Such
choices are described in the next section.

=head2 Matching this or that

Sometimes we would like to our regexp to be able to match different
possible words or character strings.  This is accomplished by using
the B<alternation> metacharacter C<|>.  To match C<dog> or C<cat>, we
form the regexp C<dog|cat>.  As before, perl will try to match the
regexp at the earliest possible point in the string.  At each
character position, perl will first try to match the first
alternative, C<dog>.  If C<dog> doesn't match, perl will then try the
next alternative, C<cat>.  If C<cat> doesn't match either, then the
match fails and perl moves to the next position in the string.  Some
examples:

    "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
    "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"

Even though C<dog> is the first alternative in the second regexp,
C<cat> is able to match earlier in the string.

    "cats"          =~ /c|ca|cat|cats/; # matches "c"
    "cats"          =~ /cats|cat|ca|c/; # matches "cats"

Here, all the alternatives match at the first string position, so the
first alternative is the one that matches.  If some of the
alternatives are truncations of the others, put the longest ones first
to give them a chance to match.

    "cab" =~ /a|b|c/ # matches "c"
                     # /a|b|c/ == /[abc]/

The last example points out that character classes are like
alternations of characters.  At a given character position, the first
alternative that allows the regexp match to succeed will be the one
that matches.

=head2 Grouping things and hierarchical matching

Alternation allows a regexp to choose among alternatives, but by
itself it unsatisfying.  The reason is that each alternative is a whole
regexp, but sometime we want alternatives for just part of a
regexp.  For instance, suppose we want to search for housecats or
housekeepers.  The regexp C<housecat|housekeeper> fits the bill, but is
inefficient because we had to type C<house> twice.  It would be nice to
have parts of the regexp be constant, like C<house>, and some
parts have alternatives, like C<cat|keeper>.

The B<grouping> metacharacters C<()> solve this problem.  Grouping
allows parts of a regexp to be treated as a single unit.  Parts of a
regexp are grouped by enclosing them in parentheses.  Thus we could solve
the C<housecat|housekeeper> by forming the regexp as
C<house(cat|keeper)>.  The regexp C<house(cat|keeper)> means match
C<house> followed by either C<cat> or C<keeper>.  Some more examples
are

    /(a|b)b/;    # matches 'ab' or 'bb'
    /(ac|b)b/;   # matches 'acb' or 'bb'
    /(^a|b)c/;   # matches 'ac' at start of string or 'bc' anywhere
    /(a|[bc])d/; # matches 'ad', 'bd', or 'cd'

    /house(cat|)/;  # matches either 'housecat' or 'house'
    /house(cat(s|)|)/;  # matches either 'housecats' or 'housecat' or
                        # 'house'.  Note groups can be nested.

    /(19|20|)\d\d/;  # match years 19xx, 20xx, or the Y2K problem, xx
    "20" =~ /(19|20|)\d\d/;  # matches the null alternative '()\d\d',
                             # because '20\d\d' can't match

Alternations behave the same way in groups as out of them: at a given
string position, the leftmost alternative that allows the regexp to
match is taken.  So in the last example at the first string position,
C<"20"> matches the second alternative, but there is nothing left over
to match the next two digits C<\d\d>.  So perl moves on to the next
alternative, which is the null alternative and that works, since
C<"20"> is two digits.

The process of trying one alternative, seeing if it matches, and
moving on to the next alternative if it doesn't, is called
B<backtracking>.  The term 'backtracking' comes from the idea that
matching a regexp is like a walk in the woods.  Successfully matching
a regexp is like arriving at a destination.  There are many possible
trailheads, one for each string position, and each one is tried in
order, left to right.  From each trailhead there may be many paths,
some of which get you there, and some which are dead ends.  When you
walk along a trail and hit a dead end, you have to backtrack along the
trail to an earlier point to try another trail.  If you hit your
destination, you stop immediately and forget about trying all the
other trails.  You are persistent, and only if you have tried all the
trails from all the trailheads and not arrived at your destination, do
you declare failure.  To be concrete, here is a step-by-step analysis
of what perl does when it tries to match the regexp

    "abcde" =~ /(abd|abc)(df|d|de)/;

=over 4

=item 0

Start with the first letter in the string 'a'.

=item 1

Try the first alternative in the first group 'abd'.

=item 2

Match 'a' followed by 'b'. So far so good.

=item 3

'd' in the regexp doesn't match 'c' in the string - a dead
end.  So backtrack two characters and pick the second alternative in
the first group 'abc'.

=item 4

Match 'a' followed by 'b' followed by 'c'.  We are on a roll
and have satisfied the first group. Set $1 to 'abc'.

=item 5

Move on to the second group and pick the first alternative
'df'.

=item 6

Match the 'd'.

=item 7

'f' in the regexp doesn't match 'e' in the string, so a dead
end.  Backtrack one character and pick the second alternative in the
second group 'd'.

=item 8

'd' matches. The second grouping is satisfied, so set $2 to
'd'.

=item 9

We are at the end of the regexp, so we are done! We have
matched 'abcd' out of the string "abcde".

=back

There are a couple of things to note about this analysis.  First, the
third alternative in the second group 'de' also allows a match, but we
stopped before we got to it - at a given character position, leftmost
wins.  Second, we were able to get a match at the first character
position of the string 'a'.  If there were no matches at the first
position, perl would move to the second character position 'b' and
attempt the match all over again.  Only when all possible paths at all
possible character positions have been exhausted does perl 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
    if ($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 string held by the highest numbered
C<$1>, C<$2>, ... that got assigned (and, somewhat related, C<$^N> to the
value of the C<$1>, C<$2>, ... most-recently assigned; i.e. the C<$1>,
C<$2>, ... associated with the rightmost closing parenthesis used in the
match).

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:

    $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