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<-]\^$> (and
-the pattern delimiter, whatever it is).
+the pattern delimiter, whatever it is).
C<]> is special because it denotes the end of a character class. C<$> 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
/[\$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 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<$x> is treated as a
variable and substituted in double quote fashion.
up and declare S<C<$string =~ /(abd|abc)(df|d|de)/;>> to be false.
Even with all this work, regexp matching happens remarkably fast. To
-speed things up, Perl compiles the regexp into a compact sequence of
-opcodes that can often fit inside a processor cache. When the code is
+speed things up, 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 Relative backreferences
Counting the opening parentheses to get the correct number for a
-backreference is errorprone as soon as there is more than one
+backreference is errorprone as soon as there is more than one
capturing group. A more convenient technique became available
with Perl 5.10: relative backreferences. To refer to the immediately
preceding capture group one now may write C<\g{-1}>, the next but
for using relative backreferences is illustrated by the following example,
where a simple pattern for matching peculiar strings is used:
- $a99a = '([a-z])(\d)\2\1'; # matches a11a, g22g, x33x, etc.
+ $a99a = '([a-z])(\d)\2\1'; # matches a11a, g22g, x33x, etc.
Now that we have this pattern stored as a handy string, we might feel
tempted to use it as a part of some other pattern:
eponymous set can be referenced. Outside of the pattern a named
capture buffer is accessible through the C<%+> hash.
-Assuming that we have to match calendar dates which may be given in one
+Assuming that we have to match calendar dates which may be given in one
of the three formats yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy, we can write
-three suitable patterns where we use 'd', 'm' and 'y' respectively as the
+three suitable patterns where we use 'd', 'm' and 'y' respectively as the
names of the buffers capturing the pertaining components of a date. The
matching operation combines the three patterns as alternatives:
}
Processing the results requires an additional if statement to determine
-whether C<$1> and C<$2> or C<$3> and C<$4> contain the goodies. It would
+whether C<$1> and C<$2> or C<$3> and C<$4> contain the goodies. It would
be easier if we could use buffer numbers 1 and 2 in second alternative as
-well, and this is exactly what the parenthesized construct C<(?|...)>,
+well, and this is exactly what the parenthesized construct C<(?|...)>,
set around an alternative achieves. Here is an extended version of the
previous pattern:
Within the alternative numbering group, buffer numbers start at the same
position for each alternative. After the group, numbering continues
-with one higher than the maximum reached across all the alteratives.
-
+with one higher than the maximum reached across all the alternatives.
=head2 Position information
=head2 Non-capturing groupings
-A group that is required to bundle a set of alternatives may or may not be
+A group that is required to bundle a set of alternatives may or may not be
useful as a capturing group. If it isn't, it just creates a superfluous
addition to the set of available capture buffer values, inside as well as
outside the regexp. Non-capturing groupings, denoted by C<(?:regexp)>,
-still allow the regexp to be treated as a single unit, but don't establish
+still allow the regexp to be treated as a single unit, but don't establish
a capturing buffer at the same time. Both capturing and non-capturing
groupings are allowed to co-exist in the same regexp. Because there is
no extraction, non-capturing groupings are faster than capturing
Whenever this is applied to a string which doesn't quite meet the
pattern's expectations such as S<C<"abc ">> or S<C<"abc def ">>,
-the regex engine will backtrack, approximately once for each character
-in the string. But we know that there is no way around taking I<all>
-of the inital word characters to match the first repetition, that I<all>
+the regex engine will backtrack, approximately once for each character
+in the string. But we know that there is no way around taking I<all>
+of the initial word characters to match the first repetition, that I<all>
spaces must be eaten by the middle part, and the same goes for the second
-word. With the introduction of the I<possessive quantifiers> in
-Perl 5.10 we have a way of instructing the regexp engine not to backtrack,
-with the usual quantifiers with a C<+> appended to them. This makes them
-greedy as well as stingy; once they succeed they won't give anything back
-to permit another solution. They have the following meanings:
+word.
+
+With the introduction of the I<possessive quantifiers> in Perl 5.10, we
+have a way of instructing the regex engine not to backtrack, with the
+usual quantifiers with a C<+> appended to them. This makes them greedy as
+well as stingy; once they succeed they won't give anything back to permit
+another solution. They have the following meanings:
=over 4
=item *
-C<a{n,m}+> means: match at least C<n> times, not more than C<m> times,
-as many times as possible, and don't give anything up. C<a?+> is short
+C<a{n,m}+> means: match at least C<n> times, not more than C<m> times,
+as many times as possible, and don't give anything up. C<a?+> is short
for C<a{0,1}+>
=item *
C<a{n,}+> means: match at least C<n> times, but as many times as possible,
-and don't give anything up. C<a*+> is short for C<a{0,}+> and C<a++> is
+and don't give anything up. C<a*+> is short for C<a{0,}+> and C<a++> is
short for C<a{1,}+>.
=item *
=back
-These possessive quantifiers represent a special case of a more general
-concept, the I<independent subexpression>, see below.
+These possessive quantifiers represent a special case of a more general
+concept, the I<independent subexpression>, see below.
As an example where a possessive quantifier is suitable we consider
matching a quoted string, as it appears in several programming languages.
The backslash is used as an escape character that indicates that the
next character is to be taken literally, as another character for the
string. Therefore, after the opening quote, we expect a (possibly
-empty) sequence of alternatives: either some character except an
+empty) sequence of alternatives: either some character except an
unescaped quote or backslash or an escaped character.
/"(?:[^"\\]++|\\.)*+"/;
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.
+extended C<//x> modifiers. There are a few more things you might
+want to know about matching operators.
=head3 Optimizing pattern evaluation
-We pointed out earlier that variables in regexps are substituted
+We pointed out earlier that variables in regexps are substituted
before the regexp is evaluated:
$pattern = 'Seuss';
print if m'@pattern'; # matches literal '@pattern', not 'Seuss'
}
-Similar to strings, C<m''> acts like apostrophes on a regexp; all other
+Similar to strings, C<m''> acts like apostrophes on a regexp; all other
C<m> delimiters act like quotes. If the regexp evaluates to the empty string,
the regexp in the I<last successful match> is used instead. So we have
=head3 The split function
The C<split()> function is another place where a regexp is used.
-C<split /regexp/, string, limit> separates the C<string> operand into
-a list of substrings and returns that list. The regexp must be designed
+C<split /regexp/, string, limit> separates the C<string> operand into
+a list of substrings and returns that list. The regexp must be designed
to match whatever constitutes the separators for the desired substrings.
-The C<limit>, if present, constrains splitting into no more than C<limit>
+The C<limit>, if present, constrains splitting into no more than C<limit>
number of strings. For example, to split a string into words, use
$x = "Calvin and Hobbes";
There are several escape sequences that convert characters or strings
between upper and lower case, and they are also available within
-patterns. C<\l> and C<\u> convert the next character to lower or
+patterns. C<\l> and C<\u> convert the next character to lower or
upper case, respectively:
$x = "perl";
The Unicode has also been separated into various sets of characters
which you can test with C<\p{...}> (in) and C<\P{...}> (not in).
To test whether a character is (or is not) an element of a script
-you would use the script name, for example C<\p{Latin}>, C<\p{Greek}>,
+you would use the script name, for example C<\p{Latin}>, C<\p{Greek}>,
or C<\P{Katakana}>. Other sets are the Unicode blocks, the names
of which begin with "In". One such block is dedicated to mathematical
operators, and its pattern formula is <C\p{InMathematicalOperators>}>.
Backtracking is more efficient than repeated tries with different regular
expressions. If there are several regular expressions and a match with
-any of them is acceptable, then it is possible to combine them into a set
+any of them is acceptable, then it is possible to combine them into a set
of alternatives. If the individual expressions are input data, this
-can be done by programming a join operation. We'll exploit this idea in
-an improved version of the C<simple_grep> program: a program that matches
+can be done by programming a join operation. We'll exploit this idea in
+an improved version of the C<simple_grep> program: a program that matches
multiple patterns:
% cat > multi_grep
Sometimes it is advantageous to construct a pattern from the I<input>
that is to be analyzed and use the permissible values on the left
hand side of the matching operations. As an example for this somewhat
-paradoxical situation, let's assume that our input contains a command
+paradoxical situation, let's assume that our input contains a command
verb which should match one out of a set of available command verbs,
-with the additional twist that commands may be abbreviated as long as
+with the additional twist that commands may be abbreviated as long as
the given string is unique. The program below demonstrates the basic
algorithm.
Rather than trying to match the input against the keywords, we match the
combined set of keywords against the input. The pattern matching
-operation S<C<$kwds =~ /\b($command\w*)/g>> does several things at the
-same time. It makes sure that the given command begins where a keyword
-begins (C<\b>). It tolerates abbreviations due to the added C<\w*>. It
-tells us the number of matches (C<scalar @matches>) and all the keywords
+operation S<C<$kwds =~ /\b($command\w*)/g>> does several things at the
+same time. It makes sure that the given command begins where a keyword
+begins (C<\b>). It tolerates abbreviations due to the added C<\w*>. It
+tells us the number of matches (C<scalar @matches>) and all the keywords
that were actually matched. You could hardly ask for more.
-
=head2 Embedding comments and modifiers in a regular expression
This style of commenting has been largely superseded by the raw,
freeform commenting that is allowed with the C<//x> modifier.
-The modifiers C<//i>, C<//m>, C<//s>, C<//x> and C<//k> (or any
+The modifiers C<//i>, C<//m>, C<//s>, C<//x> and C<//k> (or any
combination thereof) can also embedded in
a regexp using C<(?i)>, C<(?m)>, C<(?s)>, and C<(?x)>. For instance,
we have seen so far are the anchors. The anchor C<^> matches the
beginning of the line, but doesn't eat any characters. Similarly, the
word boundary anchor C<\b> matches wherever a character matching C<\w>
-is next to a character that doesn't, but it doesn't eat up any
+is next to a character that doesn't, but it doesn't eat up any
characters itself. Anchors are examples of I<zero-width assertions>.
Zero-width, because they consume
no characters, and assertions, because they test some property of the
backreference C<\integer> matched earlier in the regexp. The same
thing can be done with a name associated with a capture buffer, written
as C<< (<name>) >> or C<< ('name') >>. The second form is a bare
-zero width assertion C<(?...)>, either a lookahead, a lookbehind, or a
+zero width assertion C<(?...)>, either a lookahead, a lookbehind, or a
code assertion (discussed in the next section). The third set of forms
provides tests that return true if the expression is executed within
a recursion (C<(R)>) or is being called from some capturing group,
that the decimal fraction pattern is the first place where we can
reuse the integer pattern.
- /^ (?&osg)\ * ( (?&int)(?&dec)? | (?&dec) )
+ /^ (?&osg)\ * ( (?&int)(?&dec)? | (?&dec) )
(?: [eE](?&osg)(?&int) )?
$
(?(DEFINE)
This feature (introduced in Perl 5.10) significantly extends the
power of Perl's pattern matching. By referring to some other
capture group anywhere in the pattern with the construct
-C<(?group-ref)>, the I<pattern> within the referenced group is used
+C<(?group-ref)>, the I<pattern> within the referenced group is used
as an independent subpattern in place of the group reference itself.
Because the group reference may be contained I<within> the group it
refers to, it is now possible to apply pattern matching to tasks that
expression is denoted C<(?{code})>, with I<code> a string of Perl
statements.
-Be warned that this feature is considered experimental, and may be
+Be warned that this feature is considered experimental, and may be
changed without notice.
Code expressions are zero-width assertions, and the value they return
/^1(?:((??{ $z0 }))1(?{ $z0 = $z1; $z1 .= $^N; }))+$/
which shows that spaces are still possible in the code parts. Nevertheless,
-when working with code and conditional expressions, the extended form of
+when working with code and conditional expressions, the extended form of
regexps is almost necessary in creating and debugging regexps.
which may be abbreviated as C<(*F)>. If this is inserted in a regexp
it will cause to fail, just like at some mismatch between the pattern
and the string. Processing of the regexp continues like after any "normal"
-failure, so that, for instance, the next position in the string or another
-alternative will be tried. As failing to match doesn't preserve capture
-buffers or produce results, it may be necessary to use this in
+failure, so that, for instance, the next position in the string or another
+alternative will be tried. As failing to match doesn't preserve capture
+buffers or produce results, it may be necessary to use this in
combination with embedded code.
%count = ();
/([aeiou])(?{ $count{$1}++; })(*FAIL)/oi;
printf "%3d '%s'\n", $count{$_}, $_ for (sort keys %count);
-The pattern begins with a class matching a subset of letters. Whenever
-this matches, a statement like C<$count{'a'}++;> is executed, incrementing
-the letter's counter. Then C<(*FAIL)> does what it says, and
-the regexp engine proceeds according to the book: as long as the end of
-the string hasn't been reached, the position is advanced before looking
+The pattern begins with a class matching a subset of letters. Whenever
+this matches, a statement like C<$count{'a'}++;> is executed, incrementing
+the letter's counter. Then C<(*FAIL)> does what it says, and
+the regexp engine proceeds according to the book: as long as the end of
+the string hasn't been reached, the position is advanced before looking
for another vowel. Thus, match or no match makes no difference, and the
regexp engine proceeds until the the entire string has been inspected.
(It's remarkable that an alternative solution using something like
projects / p5sagit/p5-mst-13.2.git / commitdiff