Purpose: To document the special [Regular Expressions] escape sequences available in Tcl 8.x or newer. ---- Escapes (available only in advanced regular expressions (ARE)), 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 extended regular expressions (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: \a: alert (bell) character, as in C \b: backspace, as in C \B: synonym for \ to help reduce backslash doubling in some applications where there are multiple levels of backslash processing \cX: (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 \e: the character whose collating-sequence name is `ESC', or failing that, the character with octal value 033 \f: formfeed, as in C \n: newline, as in C \r: carriage return, as in C \t: horizontal tab, as in C \uwxyz: (where wxyz is exactly four hexadecimal digits) the Unicode character U+wxyz in the local byte ordering \Ustuvwxyz: (where stuvwxyz is exactly eight hexadecimal digits) reserved for a somewhat-hypothetical Unicode extension to 32 bits \v: vertical tab, as in C are all available. \xhhh: (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). \0: the character whose value is 0 \xy: (where xy is exactly two octal digits, and is not a back reference (see below)) the character whose octal value is 0xy \xyz: (where xyz is exactly three octal digits, and is not a back reference (see below)) the character 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: \d [[[[:digit:]]]] \s [[[[:space:]]]] \w [[[[:alnum:]]_]] (note underscore) \D [[^[[:digit:]]]] \S [[^[[:space:]]]] \W [[^[[: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: \A matches only at the beginning of the string (see MATCHING, below, for how this differs from `^') \m matches only at the beginning of a word \M matches only at the end of a word \y matches only at the beginning or end of a word \Y matches only at a point that is not the beginning or end of a word \Z matches only at the end of the string (see MATCHING, below, for how this differs from `$') \m (where m is a nonzero digit) a back reference, see below \mnn (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 \why: doesn't it format correctly? 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. ---- [Category Documentation]