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diff --git a/regex/regex.7 b/regex/regex.7 new file mode 100644 index 0000000..0fa1802 --- /dev/null +++ b/regex/regex.7 @@ -0,0 +1,235 @@ +.TH REGEX 7 "25 Oct 1995" +.BY "Henry Spencer" +.SH NAME +regex \- POSIX 1003.2 regular expressions +.SH DESCRIPTION +Regular expressions (``RE''s), +as defined in POSIX 1003.2, come in two forms: +modern REs (roughly those of +.IR egrep ; +1003.2 calls these ``extended'' REs) +and obsolete REs (roughly those of +.IR ed ; +1003.2 ``basic'' REs). +Obsolete REs mostly exist for backward compatibility in some old programs; +they will be discussed at the end. +1003.2 leaves some aspects of RE syntax and semantics open; +`\(dg' marks decisions on these aspects that +may not be fully portable to other 1003.2 implementations. +.PP +A (modern) RE is one\(dg or more non-empty\(dg \fIbranches\fR, +separated by `|'. +It matches anything that matches one of the branches. +.PP +A branch is one\(dg or more \fIpieces\fR, concatenated. +It matches a match for the first, followed by a match for the second, etc. +.PP +A piece is an \fIatom\fR possibly followed +by a single\(dg `*', `+', `?', or \fIbound\fR. +An atom followed by `*' matches a sequence of 0 or more matches of the atom. +An atom followed by `+' matches a sequence of 1 or more matches of the atom. +An atom followed by `?' matches a sequence of 0 or 1 matches of the atom. +.PP +A \fIbound\fR is `{' followed by an unsigned decimal integer, +possibly followed by `,' +possibly followed by another unsigned decimal integer, +always followed by `}'. +The integers must lie between 0 and RE_DUP_MAX (255\(dg) inclusive, +and if there are two of them, the first may not exceed the second. +An atom followed by a bound containing one integer \fIi\fR +and no comma matches +a sequence of exactly \fIi\fR matches of the atom. +An atom followed by a bound +containing one integer \fIi\fR and a comma matches +a sequence of \fIi\fR or more matches of the atom. +An atom followed by a bound +containing two integers \fIi\fR and \fIj\fR matches +a sequence of \fIi\fR through \fIj\fR (inclusive) matches of the atom. +.PP +An atom is a regular expression enclosed in `()' (matching a match for the +regular expression), +an empty set of `()' (matching the null string)\(dg, +a \fIbracket expression\fR (see below), `.' +(matching any single character), `^' (matching the null string at the +beginning of a line), `$' (matching the null string at the +end of a line), a `\e' followed by one of the characters +`^.[$()|*+?{\e' +(matching that character taken as an ordinary character), +a `\e' followed by any other character\(dg +(matching that character taken as an ordinary character, +as if the `\e' had not been present\(dg), +or a single character with no other significance (matching that character). +A `{' followed by a character other than a digit is an ordinary +character, not the beginning of a bound\(dg. +It is illegal to end an RE with `\e'. +.PP +A \fIbracket expression\fR is a list of characters enclosed in `[]'. +It normally matches any single character from the list (but see below). +If the list begins with `^', +it matches any single character +(but see below) \fInot\fR from the rest of the list. +If two characters in the list are separated by `\-', this is shorthand +for the full \fIrange\fR of characters between those two (inclusive) in the +collating sequence, +e.g. `[0\-9]' in ASCII matches any decimal digit. +It is illegal\(dg for two ranges to share an +endpoint, e.g. `a\-c\-e'. +Ranges are very collating-sequence-dependent, +and portable programs should avoid relying on them. +.PP +To include a literal `]' in the list, make it the first character +(following a possible `^'). +To include a literal `\-', make it the first or last character, +or the second endpoint of a range. +To use a literal `\-' as the first endpoint of a range, +enclose it in `[.' and `.]' to make it a collating element (see below). +With the exception of these and some combinations using `[' (see next +paragraphs), all other special characters, including `\e', lose their +special significance within a bracket expression. +.PP +Within a bracket expression, a collating element (a character, +a multi-character sequence that collates as if it were a single character, +or a collating-sequence name for either) +enclosed in `[.' and `.]' stands for the +sequence of characters of that collating element. +The sequence is a single element of the bracket expression's list. +A bracket expression containing a multi-character collating element +can thus match more than one character, +e.g. if the collating sequence includes a `ch' collating element, +then the RE `[[.ch.]]*c' matches the first five characters +of `chchcc'. +.PP +Within a bracket expression, a collating element enclosed in `[=' and +`=]' is an equivalence class, standing for the sequences of characters +of all collating elements equivalent to that one, including itself. +(If there are no other equivalent collating elements, +the treatment is as if the enclosing delimiters were `[.' and `.]'.) +For example, if o and \o'o^' are the members of an equivalence class, +then `[[=o=]]', `[[=\o'o^'=]]', and `[o\o'o^']' are all synonymous. +An equivalence class may not\(dg be an endpoint +of a range. +.PP +Within a bracket expression, the name of a \fIcharacter class\fR enclosed +in `[:' and `:]' stands for the list of all characters belonging to that +class. +Standard character class names are: +.PP +.RS +.nf +.ta 3c 6c 9c +alnum digit punct +alpha graph space +blank lower upper +cntrl print xdigit +.fi +.RE +.PP +These stand for the character classes defined in +.IR ctype (3). +A locale may provide others. +A character class may not be used as an endpoint of a range. +.PP +There are two special cases\(dg of bracket expressions: +the bracket expressions `[[:<:]]' and `[[:>:]]' match the null string at +the beginning and end of a word respectively. +A word is defined as a sequence of +word characters +which is neither preceded nor followed by +word characters. +A word character is an +.I alnum +character (as defined by +.IR ctype (3)) +or an underscore. +This is an extension, +compatible with but not specified by POSIX 1003.2, +and should be used with +caution in software intended to be portable to other systems. +.PP +In the event that an RE could match more than one substring of a given +string, +the RE matches the one starting earliest in the string. +If the RE could match more than one substring starting at that point, +it matches the longest. +Subexpressions also match the longest possible substrings, subject to +the constraint that the whole match be as long as possible, +with subexpressions starting earlier in the RE taking priority over +ones starting later. +Note that higher-level subexpressions thus take priority over +their lower-level component subexpressions. +.PP +Match lengths are measured in characters, not collating elements. +A null string is considered longer than no match at all. +For example, +`bb*' matches the three middle characters of `abbbc', +`(wee|week)(knights|nights)' matches all ten characters of `weeknights', +when `(.*).*' is matched against `abc' the parenthesized subexpression +matches all three characters, and +when `(a*)*' is matched against `bc' both the whole RE and the parenthesized +subexpression match the null string. +.PP +If case-independent matching is specified, +the effect is much as if all case distinctions had vanished from the +alphabet. +When an alphabetic that exists in multiple cases appears as an +ordinary character outside a bracket expression, it is effectively +transformed into a bracket expression containing both cases, +e.g. `x' becomes `[xX]'. +When it appears inside a bracket expression, all case counterparts +of it are added to the bracket expression, so that (e.g.) `[x]' +becomes `[xX]' and `[^x]' becomes `[^xX]'. +.PP +No particular limit is imposed on the length of REs\(dg. +Programs intended to be portable should not employ REs longer +than 256 bytes, +as an implementation can refuse to accept such REs and remain +POSIX-compliant. +.PP +Obsolete (``basic'') regular expressions differ in several respects. +`|', `+', and `?' are ordinary characters and there is no equivalent +for their functionality. +The delimiters for bounds are `\e{' and `\e}', +with `{' and `}' by themselves ordinary characters. +The parentheses for nested subexpressions are `\e(' and `\e)', +with `(' and `)' by themselves ordinary characters. +`^' is an ordinary character except at the beginning of the +RE or\(dg the beginning of a parenthesized subexpression, +`$' is an ordinary character except at the end of the +RE or\(dg the end of a parenthesized subexpression, +and `*' is an ordinary character if it appears at the beginning of the +RE or the beginning of a parenthesized subexpression +(after a possible leading `^'). +Finally, there is one new type of atom, a \fIback reference\fR: +`\e' followed by a non-zero decimal digit \fId\fR +matches the same sequence of characters +matched by the \fId\fRth parenthesized subexpression +(numbering subexpressions by the positions of their opening parentheses, +left to right), +so that (e.g.) `\e([bc]\e)\e1' matches `bb' or `cc' but not `bc'. +.SH SEE ALSO +regex(3) +.PP +POSIX 1003.2, section 2.8 (Regular Expression Notation). +.SH HISTORY +Written by Henry Spencer, based on the 1003.2 spec. +.SH BUGS +Having two kinds of REs is a botch. +.PP +The current 1003.2 spec says that `)' is an ordinary character in +the absence of an unmatched `('; +this was an unintentional result of a wording error, +and change is likely. +Avoid relying on it. +.PP +Back references are a dreadful botch, +posing major problems for efficient implementations. +They are also somewhat vaguely defined +(does +`a\e(\e(b\e)*\e2\e)*d' match `abbbd'?). +Avoid using them. +.PP +1003.2's specification of case-independent matching is vague. +The ``one case implies all cases'' definition given above +is current consensus among implementors as to the right interpretation. +.PP +The syntax for word boundaries is incredibly ugly. |