X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlre.pod;h=02dd2cda5d8260ce0421f39218d9070d90372e7c;hb=a0acbdc36d211b2eba42328df555d9ec49fa4cd4;hp=4bc042d9b3753b3214a061ef4109836dc838116f;hpb=c529f79d594c53d3968d464c57ac24a21137dd09;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlre.pod b/pod/perlre.pod index 4bc042d..02dd2cd 100644 --- a/pod/perlre.pod +++ b/pod/perlre.pod @@ -40,7 +40,7 @@ is, no matter what C<$*> contains, C without C will force "^" to match only at the beginning of the string and "$" 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 @@ -121,7 +121,7 @@ to integral values less than a preset limit defined when perl is built. This is usually 32766 on the most common platforms. The actual limit can be seen in the error message generated by code such as this: - $_ **= $_ , / {$_} / for 2 .. 42; + $_ **= $_ , / {$_} / for 2 .. 42; By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still @@ -169,7 +169,7 @@ You'll need to write something like C. 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 @@ -179,18 +179,20 @@ In addition, Perl defines the following: \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 is in effect, the list of alphabetic characters generated by C<\w> is taken from the current locale. See L. You may use C<\w>, C<\W>, C<\s>, C<\S>, -C<\d>, and C<\D> within character classes (though not as either end of -a range). See L for details about C<\pP>, C<\PP>, and C<\X>. +C<\d>, and C<\D> within character classes, but if you try to use them +as endpoints of a range, that's not a range, the "-" is understood literally. +See L for details about C<\pP>, C<\PP>, and C<\X>. The POSIX character class syntax - [:class:] + [:class:] is also available. The available classes and their backslash equivalents (if available) are as follows: @@ -198,38 +200,47 @@ equivalents (if available) are as follows: 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 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:]%] + [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 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 @@ -237,8 +248,8 @@ If the C pragma is used, the following equivalences to Unicode For example C<[:lower:]> and C<\p{IsLower}> are equivalent. If the C pragma is not used but the C 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: @@ -246,31 +257,28 @@ The assumedly non-obviously named classes are: =item cntrl - 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 control - classified as characters. +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 (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 work just fine) it is included - for completeness. - -=item +Any hexadecimal digit. Though this may feel silly ([0-9A-Fa-f] would +work just fine) it is included for completeness. =back @@ -315,23 +323,25 @@ several patterns that you want to match against consequent substrings of your string, see the previous reference. The actual location where C<\G> will match can also be influenced by using C as an lvalue. See L. - + The bracketing construct C<( ... )> creates capture buffers. To -refer to the digit'th buffer use \EdigitE within the +refer to the digit'th buffer use \ within the match. Outside the match use "$" instead of "\". (The -\EdigitE notation works in certain circumstances outside +\ notation works in certain circumstances outside the match. See the warning below about \1 vs $1 for details.) Referring back to another part of the match is called a I. 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: @@ -340,13 +350,13 @@ Examples: if (/(.)\1/) { # find first doubled char print "'$1' is the first doubled character\n"; } - + if (/Time: (..):(..):(..)/) { # parse out values $hours = $1; $minutes = $2; $seconds = $3; } - + Several special variables also refer back to portions of the previous match. C<$+> returns whatever the last bracket match matched. C<$&> returns the entire matched string. (At one point C<$0> did @@ -355,7 +365,7 @@ everything before the matched string. And C<$'> returns everything 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.) @@ -376,14 +386,15 @@ other two. Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular expression languages, there are no backslashed symbols that aren't alphanumeric. So anything -that looks like \\, \(, \), \E, \E, \{, or \} is always +that looks like \\, \(, \), \<, \>, \{, or \} is always 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 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: @@ -499,9 +510,9 @@ Sometimes it's still easier just to say: For look-behind see below. -=item C<(?E=pattern)> +=item C<(?<=pattern)> -A zero-width positive look-behind assertion. For example, C=\t)\w+/> +A zero-width positive look-behind assertion. For example, C matches a word that follows a tab, without including the tab in C<$&>. Works only for fixed-width look-behind. @@ -573,7 +584,7 @@ so you should only do so if you are also using taint checking. Better yet, use the carefully constrained evaluation within a Safe module. See L for details about both these mechanisms. -=item C<(?p{ code })> +=item C<(??{ code })> B: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice. @@ -595,12 +606,12 @@ The following pattern matches a parenthesized group: (?: (?> [^()]+ ) # Non-parens without backtracking | - (?p{ $re }) # Group with matching parens + (??{ $re }) # Group with matching parens )* \) }x; -=item C<(?Epattern)> +=item C<< (?>pattern) >> B: This extended regular expression feature is considered highly experimental, and may be changed or deleted without notice. @@ -613,7 +624,7 @@ construct is useful for optimizations of what would otherwise be It may also be useful in places where the "grab all you can, and do not give anything back" semantic is desirable. -For example: C<^(?Ea*)ab> will never match, since C<(?Ea*)> +For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> (anchored at the beginning of string, as above) will match I characters C at the beginning of string, leaving no C for C to match. In contrast, C will match the same as C, @@ -622,10 +633,10 @@ group C (see L<"Backtracking">). In particular, C inside C will match fewer characters than a standalone C, since this makes the tail match. -An effect similar to C<(?Epattern)> may be achieved by writing +An effect similar to C<< (?>pattern) >> may be achieved by writing C<(?=(pattern))\1>. This matches the same substring as a standalone C, and the following C<\1> eats the matched string; it therefore -makes a zero-length assertion into an analogue of C<(?E...)>. +makes a zero-length assertion into an analogue of C<< (?>...) >>. (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordinals of backreferences in the rest of a regular expression.) @@ -661,13 +672,14 @@ hung. However, a tiny change to this pattern \) }x -which uses C<(?E...)> matches exactly when the one above does (verifying +which uses C<< (?>...) >> matches exactly when the one above does (verifying this yourself would be a productive exercise), but finishes in a fourth the time when used on a similar string with 1000000 Cs. 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 pragma or B<-w> switch saying it +C<"matches the null string many times">): -On simple groups, such as the pattern C<(?E [^()]+ )>, a comparable +On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. This was only 4 times slower on a string with 1000000 Cs. @@ -675,7 +687,7 @@ The "grab all you can, and do not give anything back" semantic is desirable 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 the correct subexpression to match +its appearance, C<#[ \t]*> I 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: @@ -717,6 +729,11 @@ themselves. =head2 Backtracking +NOTE: This section presents an abstract approximation of regular +expression behavior. For a more rigorous (and complicated) view of +the rules involved in selecting a match among possible alternatives, +see L. + A fundamental feature of regular expression matching involves the notion called I, which is currently used (when needed) by all regular expression quantifiers, namely C<*>, C<*?>, C<+>, @@ -898,19 +915,23 @@ ways they can use backtracking to try match. For example, without 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", -which does not backtrack (see Lpattern)>>). Note also that +which does not backtrack (see Lpattern) >>>). Note also that zero-length look-ahead/look-behind assertions will not backtrack to make the tail match, since they are in "logical" context: only whether they match is considered relevant. For an example where side-effects of look-ahead I have influenced the -following match, see Lpattern)>>. +following match, see Lpattern) >>>. =head2 Version 8 Regular Expressions @@ -930,11 +951,16 @@ in C<[]>, which will match any one character from the list. If the first character after the "[" is "^", the class matches any character not in the list. Within a list, the "-" character specifies a range, so that C represents all characters between "a" and "z", -inclusive. If you want "-" itself to be a member of a class, put it -at the start or end of the list, or escape it with a backslash. (The +inclusive. If you want either "-" or "]" itself to be a member of a +class, put it at the start of the list (possibly after a "^"), or +escape it with a backslash. "-" is also taken literally when it is +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.) +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 @@ -946,11 +972,11 @@ spell out the character sets in full. 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, where I is a string -of octal digits, matches the character whose ASCII value is I. -Similarly, \xI, where I are hexadecimal digits, matches the -character whose ASCII value is I. The expression \cI matches the -ASCII character control-I. Finally, the "." metacharacter matches any -character except "\n" (unless you use C). +of octal digits, matches the character whose coded character set value +is I. Similarly, \xI, where I are hexadecimal digits, +matches the character whose numeric value is I. The expression \cI +matches the character control-I. Finally, the "." metacharacter +matches any character except "\n" (unless you use C). You can specify a series of alternatives for a pattern using "|" to separate them, so that C will match any of "fee", "fie", @@ -1072,7 +1098,7 @@ For example: $_ = 'bar'; s/\w??/<$&>/g; -results in C<"<><><><>">. At each position of the string the best +results in C<< <><><><> >>. At each position of the string the best match given by non-greedy C is the zero-length match, and the I match is what is matched by C<\w>. Thus zero-length matches alternate with one-character-long matches. @@ -1085,6 +1111,107 @@ the matched string, and is reset by each assignment to pos(). Zero-length matches at the end of the previous match are ignored during C. +=head2 Combining pieces together + +Each of the elementary pieces of regular expressions which were described +before (such as C or C<\Z>) could match at most one substring +at the given position of the input string. However, in a typical regular +expression these elementary pieces are combined into more complicated +patterns using combining operators C, C, C etc +(in these examples C and C are regular subexpressions). + +Such combinations can include alternatives, leading to a problem of choice: +if we match a regular expression C against C<"abc">, will it match +substring C<"a"> or C<"ab">? One way to describe which substring is +actually matched is the concept of backtracking (see L<"Backtracking">). +However, this description is too low-level and makes you think +in terms of a particular implementation. + +Another description starts with notions of "better"/"worse". All the +substrings which may be matched by the given regular expression can be +sorted from the "best" match to the "worst" match, and it is the "best" +match which is chosen. This substitutes the question of "what is chosen?" +by the question of "which matches are better, and which are worse?". + +Again, for elementary pieces there is no such question, since at most +one match at a given position is possible. This section describes the +notion of better/worse for combining operators. In the description +below C and C are regular subexpressions. + +=over 4 + +=item C + +Consider two possible matches, C and C, C and C are +substrings which can be matched by C, C and C are substrings +which can be matched by C. + +If C is better match for C than C, C is a better +match than C. + +If C and C coincide: C is a better match than C if +C is better match for C than C. + +=item C + +When C can match, it is a better match than when only C can match. + +Ordering of two matches for C is the same as for C. Similar for +two matches for C. + +=item C + +Matches as C (repeated as many times as necessary). + +=item C + +Matches as C. + +=item C + +Matches as C. + +=item C, C, C + +Same as C, C, C respectively. + +=item C, C, C + +Same as C, C, C respectively. + +=item C<< (?>S) >> + +Matches the best match for C and only that. + +=item C<(?=S)>, C<(?<=S)> + +Only the best match for C is considered. (This is important only if +C has capturing parentheses, and backreferences are used somewhere +else in the whole regular expression.) + +=item C<(?!S)>, C<(? + +For this grouping operator there is no need to describe the ordering, since +only whether or not C can match is important. + +=item C<(??{ EXPR })> + +The ordering is the same as for the regular expression which is +the result of EXPR. + +=item C<(?(condition)yes-pattern|no-pattern)> + +Recall that which of C or C actually matches is +already determined. The ordering of the matches is the same as for the +chosen subexpression. + +=back + +The above recipes describe the ordering of matches I. +One more rule is needed to understand how a match is determined for the +whole regular expression: a match at an earlier position is always better +than a match at a later position. + =head2 Creating custom RE engines Overloaded constants (see L) provide a simple way to extend @@ -1153,5 +1280,7 @@ L. L. +L. + I by Jeffrey Friedl, published by O'Reilly and Associates.