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

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.