◆DIFFERENT FLAVORS OF REs（和标准正则表达式的区别） Regular expressions, as defined by POSIX, come in two flavors: extended REs and basic REs. EREs are roughly those of the traditional egrep, while BREs are roughly those of the traditional ed. This implementation adds a third flavor, advanced REs, basically EREs with some significant extensions. This manual page primarily describes AREs. BREs mostly exist for backward compatibility in some old programs; they will be discussed at the end. POSIX EREs are almost an exact subset of AREs. Features of AREs that are not present in EREs will be indicated. ◆REGULAR EXPRESSION SYNTAX（语法） Tcl regular expressions are implemented using the package written by Henry Spencer, based on the 1003.2 spec and some (not quite all) of the Perl5 extensions (thanks, Henry!). Much of the description of regular expressions below is copied verbatim from his manual entry. An ARE is one or more branches, separated by `|', matching anything that matches any of the branches. A branch is zero or more constraints or quantified atoms, concatenated. It matches a match for the first, followed by a match for the second, etc; an empty branch matches the empty string. A quantified atom is an atom possibly followed by a single quantifier. Without a quantifier, it matches a match for the atom. The quantifiers, and what a so-quantified atom matches, are: 字符 * + ? {m} {m,} {m,n} {m,}? {m,n}? 意义 a sequence of 0 or more matches of the atom a sequence of 1 or more matches of the atom a sequence of 0 or 1 matches of the atom a sequence of exactly m matches of the atom a sequence of m or more matches of the atom a sequence of m through n (inclusive) matches of the atom; m may not exceed n rather than the largest number of matches (see MATCHING) *? +? ?? {m}? non-greedy quantifiers, which match the same possibilities, but prefer the smallest number The forms using { and } are known as bounds. The numbers m and n are unsigned decimal integers with permissible values from 0 to 255 inclusive. An atom is one of: 字符 (re) (?:re) () 意义 (where re is any regular expression) matches a match for re, with the match noted for possible reporting as previous, but does no reporting matches an empty string, noted for possible reporting (?:) [chars] . \\k \\c { x matches an empty string, without reporting a bracket expression, matching any one of the chars (see BRACKET EXPRESSIONS for more detail) matches any single character where k is a non-alphanumeric character) matches that character taken as an ordinary character, e.g. \\\\ matches a backslash character where c is alphanumeric (possibly followed by other characters), an escape (AREs only), see ESCAPES below when followed by a character other than a digit, matches the left-brace character `{'; when followed by a digit, it is the beginning of a bound (see above) where x is a single character with no other significance, matches that character. A constraint matches an empty string when specific conditions are met. A constraint may not be followed by a quantifier. The simple constraints are as follows; some more constraints are described later, under ESCAPES. 字符 ^ $ (?=re) (?!re) 意义 matches at the beginning of a line matches at the end of a line positive lookahead (AREs only), matches at any point where a substring matching re begins negative lookahead (AREs only), matches at any point where no substring matching re begins The lookahead constraints may not contain back references (see later), and all parentheses within them are considered non-capturing. An RE may not end with `\\'. ◆BRACKET EXPRESSIONS（预定义表达式） A bracket expression 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) not from the rest of the list. If two characters in the list are separated by `-', this is shorthand for the full range of characters between those two (inclusive) in the collating sequence, e.g. [0-9] in ASCII matches any decimal digit. Two ranges may not share an endpoint, so e.g. a-c-e is illegal. Ranges are very collating-sequence-dependent, and portable programs should avoid relying on them. To include a literal ] or - in the list, the simplest method is to enclose it in [. and .] to make it a collating element (see below). Alternatively, make it the first character (following a possible `^'), or (AREs only) precede it with `\\'. Alternatively, for `-', make it the last character, or the second endpoint of a range. To use a literal - as the first endpoint of a range, make it a collating element or (AREs only) precede it with `\\'. With the exception of these, some combinations using [ (see next paragraphs), and escapes, all other special characters lose their special significance within a bracket expression. 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 in a locale that has multi-character collating elements can thus match more than one character. So (insidiously), a bracket expression that starts with ^ can match multi-character collating elements even if none of them appear in the bracket expression! (Note: Tcl currently has no multi-character collating elements. This information is only for illustration.) For example, assume the collating sequence includes a ch multi-character collating element. Then the RE [[.ch.]]*c (zero or more ch's followed by c) matches the first five characters of `chchcc'. Also, the RE [^c]b matches all of `chb' (because [^c] matches the multi-character ch). 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 ?are the members of an equivalence class, then `[[=o=]]', `[[=?]]', and `[o' are all synonymous. An equivalence class may not be an endpoint of a range. (Note: Tcl currently implements only the Unicode locale. It doesn't define any equivalence classes. The examples above are just illustrations.) Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of all characters (not all collating elements!) belonging to that class. Standard character classes are: 字符 alpha upper lower digit xdigit alnum print blank space punct graph cntrl 意义 A letter. An upper-case letter. A lower-case letter. A decimal digit. A hexadecimal digit. An alphanumeric (letter or digit). An alphanumeric (same as alnum). A space or tab character. A character producing white space in displayed text. A punctuation character. A character with a visible representation. A control character. A locale may provide others. (Note that the current Tcl implementation has only one locale: the Unicode locale.) A character class may not be used as an endpoint of a range. There are two special cases of bracket expressions: the bracket expressions [[:<:]] and [[:>:]] are constraints, matching empty strings at the beginning and end of a word respectively. A word is defined as a sequence of word characters that is neither preceded nor followed by word characters. A word character is an alnum character or an underscore (_). These special bracket expressions are deprecated; users of AREs should use constraint escapes instead (see below). ◆ESCAPES（转意字符） Escapes (AREs only), which begin with a \\ followed by an alphanumeric character, come in several varieties: character entry, class shorthands, constraint escapes, and back references. A \\ followed by an alphanumeric character but not constituting a valid escape is illegal in AREs. In EREs, there are no escapes: outside a bracket expression, a \\ followed by an alphanumeric character merely stands for that character as an ordinary character, and inside a bracket expression, \\ is an ordinary character. (The latter is the one actual incompatibility between EREs and AREs.) Character-entry escapes (AREs only) exist to make it easier to specify non-printing and otherwise inconvenient characters in REs: 字符 \\a \\b \\B \\cX \\e \\f \\n \\r \\t \%uwxyz \\Ustuvwxyz \\v \\xhhh \\0 \\xy \\xyz 意义 alert (bell) character, as in C backspace, as in C synonym for \\ to help reduce backslash doubling in some applications where there are multiple levels of backslash processing (where X is any character) the character whose low-order 5 bits are the same as those of X, and whose other bits are all zero the character whose collating-sequence name is `ESC', or failing that, the character with octal value 033 formfeed, as in C newline, as in C carriage return, as in C horizontal tab, as in C (where wxyz is exactly four hexadecimal digits) the Unicode character U+wxyz in the local byte ordering (where stuvwxyz is exactly eight hexadecimal digits) reserved for a somewhat-hypothetical Unicode extension to 32 bits vertical tab, as in C are all available. (where hhh is any sequence of hexadecimal digits) the character whose hexadecimal value is 0xhhh (a single character no matter how many hexadecimal digits are used). the character whose value is 0 (where xy is exactly two octal digits, and is not a back reference (see below)) the character whose octal value is 0xy (where xyz is exactly three octal digits, and is not a back reference (see below)) the character