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.Then bit 6 of the character (hex 40) is inverted.Thus "\cz" becomes hex 1A, but "\c{" becomeshex 3B, while "\c;" becomes hex 7B.After "\x", up to two hexadecimal digits are read (letters can be in upper or lower case).After "\0" up to two further octal digits are read.In both cases, if there are fewer than two digits,just those that are present are used.Thus the sequence "\0\x\07" specifies two binary zerosfollowed by a BEL character.Make sure you supply two digits after the initial zero if the characterthat follows is itself an octal digit.The handling of a backslash followed by a digit other than 0 is complicated.Outside a characterclass, PCRE reads it and any following digits as a decimal number.If the number is less than 10, orif there have been at least that many previous capturing left parentheses in the expression, the entiresequence is taken as a back reference.A description of how this works is given later, following thediscussion of parenthesized subpatterns.Inside a character class, or if the decimal number is greater than 9 and there have not been that manycapturing subpatterns, PCRE re-reads up to three octal digits follow- ing the backslash, andgenerates a single byte from the least significant 8 bits of the value.Any subsequent digits stand forthemselves.For example:\040is another way of writing a space\40is the same, provided there are fewer than 40 previous capturing subpatterns\7is always a back reference\11might be a back reference, or another way of writing a tab\011is always a tab\0113is a tab followed by the character "3"1427PCRE\113is the character with octal code 113 (since there can be no more than 99 back references)\377is a byte consisting entirely of 1 bits\81is either a back reference, or a binary zero followed by the two characters "8" and "1"Note that octal values of 100 or greater must not be intro- duced by a leading zero, because no morethan three octal digits are ever read.All the sequences that define a single byte value can be used both inside and outside characterclasses.In addition, inside a character class, the sequence "\b" is interpreted as the backspacecharacter (hex 08).Outside a character class it has a different meaning (see below).The third use of backslash is for specifying generic charac- ter types:\dany decimal digit\Dany character that is not a decimal digit\sany whitespace character\Sany character that is not a whitespace character\wany "word" character\Wany "non-word" characterEach pair of escape sequences partitions the complete set of characters into two disjoint sets.Anygiven character matches one, and only one, of each pair.A "word" character is any letter or digit or the underscore character, that is, any character which canbe part of a Perl "word".The definition of letters and digits is controlled by PCRE s character tables,and may vary if locale-specific matching is taking place (see "Locale support" above).For example,in the "fr" (French) locale, some char- acter codes greater than 128 are used for accented letters, andthese are matched by\w.These character type sequences can appear both inside and outside character classes.They eachmatch one character of the appropriate type.If the current matching point is at the end of the subjectstring, all of them fail, since there is no character to match.The fourth use of backslash is for certain simple asser- tions.An assertion specifies a condition thathas to be met at a particular point in a match, without consuming any characters from the subject1428PCREstring.The use of subpatterns for more complicated assertions is described below.The backslashedassertions are\bword boundary\Bnot a word boundary\Astart of subject (independent of multiline mode)\Zend of subject or newline at end (independent of multiline mode)\zend of subject (independent of multiline mode)These assertions may not appear in character classes (but note that "\b" has a different meaning,namely the backspace character, inside a character class).A word boundary is a position in the subject string where the current character and the previouscharacter do not both match\wor\W(i.e.one matches\wand the other matches\W), or the start orend of the string if the first or last character matches \w, respectively.The\A,\Z, and\zassertions differ from the traditional circumflex and dollar (described below) inthat they only ever match at the very start and end of the subject string, whatever options are set.They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options.The difference between\Zand\zis that\Zmatches before a newline that is the last character of the string as well as at theend of the string, whereas\zmatches only at the end.Circumflex and dollarOutside a character class, in the default matching mode, thecircumflex character is an assertion which is true only ifthe current matching point is at the start of the subjectstring.Inside a character class, circumflex has an entirelydifferent meaning (see below).Circumflex need not be the first character of the pattern ifa number of alternatives are involved, but it should be thefirst thing in each alternative in which it appears if thepattern is ever to match that branch.If all possible alter-natives start with a circumflex, that is, if the pattern isconstrained to match only at the start of the subject, it issaid to be an "anchored" pattern.(There are also other con-structs that can cause a pattern to be anchored.)A dollar character is an assertion which isTRUEonly if thecurrent matching point is at the end of the subject string,1429PCREor immediately before a newline character that is the lastcharacter in the string (by default).Dollar need not be thelast character of the pattern if a number of alternativesare involved, but it should be the last item in any branchin which it appears.Dollar has no special meaning in acharacter class.The meaning of dollar can be changed so that it matches onlyat the very end of the string, by setting thePCRE_DOLLAR_ENDONLY option at compile or matching time.Thisdoes not affect the \Z assertion.The meanings of the circumflex and dollar characters arechanged if the PCRE_MULTILINE option is set.When this isthe case, they match immediately after and immediatelybefore an internal "\n" character, respectively, in additionto matching at the start and end of the subject string.Forexample, the pattern /^abc$/ matches the subject string"def\nabc" in multiline mode, but not otherwise.Conse-quently, patterns that are anchored in single line modebecause all branches start with "^" are not anchored in mul-tiline mode.The PCRE_DOLLAR_ENDONLY option is ignored ifPCRE_MULTILINE is set.Note that the sequences \A, \Z, and \z can be used to matchthe start and end of the subject in both modes, and if allbranches of a pattern start with \A is it always anchored,whether PCRE_MULTILINE is set or not.FULL STOPOutside a character class, a dot in the pattern matches anyone character in the subject, including a non-printingcharacter, but not (by default) newline.If the PCRE_DOTALLoption is set, then dots match newlines as well.The han-dling of dot is entirely independent of the handling of cir-cumflex and dollar, the only relationship being that theyboth involve newline characters.Dot has no special meaningin a character class.Square bracketsAn opening square bracket introduces a character class, ter-minated by a closing square bracket.A closing squarebracket on its own is not special.If a closing squarebracket is required as a member of the class, it should bethe first data character in the class (after an initial cir-1430PCREcumflex, if present) or escaped with a backslash [ Pobierz całość w formacie PDF ]