"^" to match only at the beginning of the string and "$" to match
only at the end (or just before a newline at the end) of the string.
Together, as /ms, they let the "." match any character whatsoever,
-while yet allowing "^" and "$" to match, respectively, just after
+while still allowing "^" and "$" to match, respectively, just after
and just before newlines within the string.
=item x
In addition, Perl defines the following:
\w Match a "word" character (alphanumeric plus "_")
- \W Match a non-word character
+ \W Match a non-"word" character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a digit character
\X Match eXtended Unicode "combining character sequence",
equivalent to C<(?:\PM\pM*)>
\C Match a single C char (octet) even under utf8.
+ (Currently this does not work correctly.)
-A C<\w> matches a single alphanumeric character, not a whole word.
+A C<\w> matches a single alphanumeric character or C<_>, not a whole word.
Use C<\w+> to match a string of Perl-identifier characters (which isn't
the same as matching an English word). If C<use locale> is in effect, the
list of alphabetic characters generated by C<\w> is taken from the
alpha
alnum
ascii
+ blank [1]
cntrl
digit \d
graph
lower
print
punct
- space \s
+ space \s [2]
upper
- word \w
+ word \w [3]
xdigit
+ [1] A GNU extension equivalent to C<[ \t]>, `all horizontal whitespace'.
+ [2] Not I<exactly equivalent> to C<\s> since the C<[[:space:]]> includes
+ also the (very rare) `vertical tabulator', "\ck", chr(11).
+ [3] A Perl extension.
+
For example use C<[:upper:]> to match all the uppercase characters.
-Note that the C<[]> are part of the C<[::]> construct, not part of the whole
-character class. For example:
+Note that the C<[]> are part of the C<[::]> construct, not part of the
+whole character class. For example:
[01[:alpha:]%]
-matches one, zero, any alphabetic character, and the percentage sign.
+matches zero, one, any alphabetic character, and the percentage sign.
If the C<utf8> pragma is used, the following equivalences to Unicode
-\p{} constructs hold:
+\p{} constructs and equivalent backslash character classes (if available),
+will hold:
alpha IsAlpha
alnum IsAlnum
ascii IsASCII
+ blank IsSpace
cntrl IsCntrl
- digit IsDigit
+ digit IsDigit \d
graph IsGraph
lower IsLower
print IsPrint
punct IsPunct
space IsSpace
+ IsSpacePerl \s
upper IsUpper
word IsWord
xdigit IsXDigit
For example C<[:lower:]> and C<\p{IsLower}> are equivalent.
If the C<utf8> pragma is not used but the C<locale> pragma is, the
-classes correlate with the isalpha(3) interface (except for `word',
-which is a Perl extension, mirroring C<\w>).
+classes correlate with the usual isalpha(3) interface (except for
+`word' and `blank').
The assumedly non-obviously named classes are:
Any control character. Usually characters that don't produce output as
such but instead control the terminal somehow: for example newline and
backspace are control characters. All characters with ord() less than
-32 are most often classified as control characters.
+32 are most often classified as control characters (assuming ASCII,
+the ISO Latin character sets, and Unicode).
=item graph
-Any alphanumeric or punctuation character.
+Any alphanumeric or punctuation (special) character.
=item print
-Any alphanumeric or punctuation character or space.
+Any alphanumeric or punctuation (special) character or space.
=item punct
-Any punctuation character.
+Any punctuation (special) character.
=item xdigit
-Any hexadecimal digit. Though this may feel silly (/0-9a-f/i would
+Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would
work just fine) it is included for completeness.
=back
There is no limit to the number of captured substrings that you may
use. However Perl also uses \10, \11, etc. as aliases for \010,
-\011, etc. (Recall that 0 means octal, so \011 is the 9'th ASCII
-character, a tab.) Perl resolves this ambiguity by interpreting
-\10 as a backreference only if at least 10 left parentheses have
-opened before it. Likewise \11 is a backreference only if at least
-11 left parentheses have opened before it. And so on. \1 through
-\9 are always interpreted as backreferences."
+\011, etc. (Recall that 0 means octal, so \011 is the character at
+number 9 in your coded character set; which would be the 10th character,
+a horizontal tab under ASCII.) Perl resolves this
+ambiguity by interpreting \10 as a backreference only if at least 10
+left parentheses have opened before it. Likewise \11 is a
+backreference only if at least 11 left parentheses have opened
+before it. And so on. \1 through \9 are always interpreted as
+backreferences.
Examples:
after the matched string.
The numbered variables ($1, $2, $3, etc.) and the related punctuation
-set (C<<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped
+set (C<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped
until the end of the enclosing block or until the next successful
match, whichever comes first. (See L<perlsyn/"Compound Statements">.)
interpreted as a literal character, not a metacharacter. This was
once used in a common idiom to disable or quote the special meanings
of regular expression metacharacters in a string that you want to
-use for a pattern. Simply quote all non-alphanumeric characters:
+use for a pattern. Simply quote all non-"word" characters:
$pattern =~ s/(\W)/\\$1/g;
+(If C<use locale> is set, then this depends on the current locale.)
Today it is more common to use the quotemeta() function or the C<\Q>
metaquoting escape sequence to disable all metacharacters' special
meanings like this:
this yourself would be a productive exercise), but finishes in a fourth
the time when used on a similar string with 1000000 C<a>s. Be aware,
however, that this pattern currently triggers a warning message under
-B<-w> saying it C<"matches the null string many times">):
+the C<use warnings> pragma or B<-w> switch saying it
+C<"matches the null string many times">):
On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable
effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>.
in many situations where on the first sight a simple C<()*> looks like
the correct solution. Suppose we parse text with comments being delimited
by C<#> followed by some optional (horizontal) whitespace. Contrary to
-its appearence, C<#[ \t]*> I<is not> the correct subexpression to match
+its appearance, C<#[ \t]*> I<is not> the correct subexpression to match
the comment delimiter, because it may "give up" some whitespace if
the remainder of the pattern can be made to match that way. The correct
answer is either one of these:
internal optimizations done by the regular expression engine, this will
take a painfully long time to run:
- 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/
+ 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/
-And if you used C<*>'s instead of limiting it to 0 through 5 matches,
-then it would take forever--or until you ran out of stack space.
+And if you used C<*>'s in the internal groups instead of limiting them
+to 0 through 5 matches, then it would take forever--or until you ran
+out of stack space. Moreover, these internal optimizations are not
+always applicable. For example, if you put C<{0,5}> instead of C<*>
+on the external group, no current optimization is applicable, and the
+match takes a long time to finish.
A powerful tool for optimizing such beasts is what is known as an
"independent group",
at the end of the list, just before the closing "]". (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.)
-Also, if you try to use the character classes C<\w>, C<\W>, C<\s>,
-C<\S>, C<\d>, or C<\D> as endpoints of a range, that's not a range,
-the "-" is understood literally.
+specifies a class containing twenty-six characters, even on EBCDIC
+based coded character sets.) Also, if you try to use the character
+classes C<\w>, C<\W>, C<\s>, C<\S>, C<\d>, or C<\D> as endpoints of
+a range, that's not a range, the "-" is understood literally.
Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
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>).
+of octal digits, matches the character whose coded character set value
+is I<nnn>. Similarly, \xI<nn>, where I<nn> are hexadecimal digits,
+matches the character whose numeric value is I<nn>. The expression \cI<x>
+matches the 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",
$_ = 'bar';
s/\w??/<$&>/g;
-results in C<"<><b><><a><><r><>">. At each position of the string the best
+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.
notion of better/worse for combining operators. In the description
below C<S> and C<T> are regular subexpressions.
-=over
+=over 4
=item C<ST>
L<perllocale>.
+L<perlebcdic>.
+
I<Mastering Regular Expressions> by Jeffrey Friedl, published
by O'Reilly and Associates.
projects / p5sagit/p5-mst-13.2.git / blobdiff