whose octal value is 0xyz Hexadecimal digits are `0'-`9', `a'-`f', and `A'-`F'. Octal digits are `0'-`7'. The character-entry escapes are always taken as ordinary characters. For example, \\135 is ] in ASCII, but \\135 does not terminate a bracket expression. Beware, however, that some applications (e.g., C compilers) interpret such sequences themselves before the regular-expression package gets to see them, which may require doubling (quadrupling, etc.) the `\\'. Class-shorthand escapes (AREs only) provide shorthands for certain commonly-used character classes: 缩写 \\d \\s \\w \\D \\S \\W 代表的完整表达式 [[:digit:]] [[:space:]] [[:alnum:]_] (note underscore) [^[:digit:]] [^[:space:]] [^[:alnum:]_] (note underscore) Within bracket expressions, `\\d', `\\s', and `\\w' lose their outer brackets, and `\\D', `\\S', and `\\W' are illegal. (So, for example, [a-c\\d] is equivalent to [a-c[:digit:]]. Also, [a-c\\D], which is equivalent to [a-c^[:digit:]], is illegal.) A constraint escape (AREs only) is a constraint, matching the empty string if specific conditions are met, written as an escape: 字符 \\A \\m \\M \\y \\Y \\Z \\m \\mnn 意义 matches only at the beginning of the string (see MATCHING, below, for how this differs from `^') matches only at the beginning of a word matches only at the end of a word matches only at the beginning or end of a word matches only at a point that is not the beginning or end of a word matches only at the end of the string (see MATCHING, below, for how this differs from `$') (where m is a nonzero digit) a back reference, see below (where m is a nonzero digit, and nn is some more digits, and the decimal value mnn is not greater than the number of closing capturing parentheses seen so far) a back reference, see below A word is defined as in the specification of [[:<:]] and [[:>:]] above. Constraint escapes are illegal within bracket expressions. A back reference (AREs only) matches the same string matched by the parenthesized subexpression specified by the number, so that (e.g.) ([bc])\\1 matches bb or cc but not `bc'. The subexpression must entirely precede the back reference in the RE. Subexpressions are numbered in the order of their leading parentheses. Non-capturing parentheses do not define subexpressions. There is an inherent historical ambiguity between octal character-entry escapes and back references, which is resolved by heuristics, as hinted at above. A leading zero always indicates an octal escape. A single non-zero digit, not followed by another digit, is always taken as a back reference. A multi-digit sequence not starting with a zero is taken as a back reference if it comes after a suitable subexpression (i.e. the number is in the legal range for a back reference), and otherwise is taken as octal. ◆METASYNTAX（内嵌语法） In addition to the main syntax described above, there are some special forms and miscellaneous syntactic facilities available. Normally the flavor of RE being used is specified by application-dependent means. However, this can be overridden by a director. If an RE of any flavor begins with `***:', the rest of the RE is an ARE. If an RE of any flavor begins with `***=', the rest of the RE is taken to be a literal string, with all characters considered ordinary characters. An ARE may begin with embedded options: a sequence (?xyz) (where xyz is one or more alphabetic characters) specifies options affecting the rest of the RE. These supplement, and can override, any options specified by the application. The available option letters are: 字符 b c e i m n p q s t w x 意义 rest of RE is a BRE case-sensitive matching (usual default) rest of RE is an ERE case-insensitive matching (see MATCHING, below) historical synonym for n newline-sensitive matching (see MATCHING, below) partial newline-sensitive matching (see MATCHING, below) rest of RE is a literal string, all ordinary characters non-newline-sensitive matching (usual default) tight syntax (usual default; see below) inverse partial newline-sensitive matching (see MATCHING, below) expanded syntax (see below) Embedded options take effect at the ) terminating the sequence. They are available only at the start of an ARE, and may not be used later within it. In addition to the usual (tight) RE syntax, in which all characters are significant, there is an expanded syntax, available in all flavors of RE with the -expanded switch, or in AREs with the embedded x option. In the expanded syntax, white-space characters are ignored and all characters between a # and the following newline (or the end of the RE) are ignored, permitting paragraphing and commenting a complex RE. There are three exceptions to that basic rule: a white-space character or `#' preceded by `\\' is retained white space or `#' within a bracket expression is retained
white space and comments are illegal within multi-character symbols like the ARE `(?:' or the BRE `\\('
Expanded-syntax white-space characters are blank, tab, newline, and any character that belongs to the space character class.
Finally, in an ARE, outside bracket expressions, the sequence `(?#ttt)' (where ttt is any text not containing a `)') is a comment, completely ignored. Again, this is not allowed between the characters of multi-character symbols like `(?:'. Such comments are more a historical artifact than a useful facility, and their use is deprecated; use the expanded syntax instead.
None of these metasyntax extensions is available if the application (or an initial ***= director) has specified that the user's input be treated as a literal string rather than as an RE.
◆MATCHING（匹配）
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, its choice is determined by its preference: either the longest substring, or the shortest.
Most atoms, and all constraints, have no preference. A parenthesized RE has the same preference (possibly none) as the RE. A quantified atom with quantifier {m} or {m}? has the same preference (possibly none) as the atom itself. A quantified atom with other normal quantifiers (including {m,n} with m equal to n) prefers longest match. A quantified atom with other non-greedy quantifiers (including {m,n}? with m equal to n) prefers shortest match. A branch has the same preference as the first quantified atom in it which has a preference. An RE consisting of two or more branches connected by the | operator prefers longest match.
Subject to the constraints imposed by the rules for matching the whole RE, subexpressions also match the longest or shortest possible substrings, based on their preferences, with subexpressions starting earlier in the RE taking priority over ones starting later. Note that outer subexpressions thus take priority over their component subexpressions.
Note that the quantifiers {1,1} and {1,1}? can be used to force longest and shortest preference, respectively, on a subexpression or a whole RE.
Match lengths are measured in characters, not collating elements. An empty string is considered longer than no match at all. For example, bb* matches the three middle characters of `abbbc', (week|wee)(night|knights) 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 an empty string.
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, so that x becomes `[xX]'. When it appears inside a bracket expression, all case counterparts of it are added to the bracket expression, so that [x] becomes [xX] and [^x] becomes `[^xX]'.
If newline-sensitive matching is specified, . and bracket expressions using ^ will never match the newline character (so that matches will never cross newlines unless the RE explicitly arranges it) and ^ and $ will match the empty string after and before a newline respectively, in addition to matching at beginning and end of string respectively. ARE \\A and \\Z continue to match beginning or end of string only.
If partial newline-sensitive matching is specified, this affects . and bracket expressions as with newline-sensitive matching, but not ^ and `$'.
If inverse partial newline-sensitive matching is specified, this affects ^ and $ as with newline-sensitive matching, but not . and bracket expressions. This isn't very useful but is provided for symmetry.
◆LIMITS AND COMPATIBILITY（限制和兼容性）
No particular limit is imposed on the length of REs. Programs intended to be highly portable should not employ REs longer than 256 bytes, as a POSIX-compliant implementation can refuse to accept such REs.
The only feature of AREs that is actually incompatible with POSIX EREs is that \\ does not lose its special significance inside bracket expressions. All other ARE features use syntax which is illegal or has undefined or unspecified effects in POSIX EREs; the *** syntax of directors likewise is outside the POSIX syntax for both BREs and EREs. Many of the ARE extensions are borrowed from Perl, but some have been changed to clean them up, and a few Perl extensions are not present. Incompatibilities of note include `\\b', `\\B', the lack of special treatment for a trailing newline, the addition of complemented bracket expressions to the things affected by newline-sensitive matching, the restrictions on parentheses and back references in lookahead constraints, and the longest/shortest-match (rather than first-match) matching semantics.
The matching rules for REs containing both normal and non-greedy quantifiers have changed since early beta-test versions of this package. (The new rules are much simpler and cleaner, but don't work as hard at guessing the user's real intentions.)
Henry Spencer's original 1986 regexp package, still in widespread use (e.g., in pre-8.1 releases of Tcl), implemented an early version of today's EREs. There are four incompatibilities between regexp's near-EREs (`RREs' for short) and AREs. In roughly increasing order of significance:
In AREs, \\ followed by an alphanumeric character is either an escape or an error, while in RREs, it was just another way of writing the alphanumeric. This should not be a problem because there was no reason to write such a sequence in RREs.
{ followed by a digit in an ARE is the beginning of a bound, while in RREs, { was always an ordinary character. Such sequences should be rare, and will often result in an error because following characters will not look like a valid bound.
In AREs, \\ remains a special character within `[]', so a literal \\ within [] must be written `\\\\'. \\\\ also gives a literal