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:
\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 zerofailing that, the character with octal value 033
\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 charac-
ters. For example, \135 is ] in ASCII, but \135 does not ter-
minate a bracket expression. Beware, however, that some applica-
tions (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 cer-
tain 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 exam- |
ple, [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 MATCH-
ING, 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 expres-
sions.
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 subexpres-
sion must entirely precede the back reference in the RE. Subex-
pressions 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 refer-
ence 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.
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