Syntax of the builtin regular expression library
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:
|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:
Within bracket expressions, `d', `s', and `w' lose their outer brackets, and `D', `S', and `W' are illegal. (So, for example, a-cd is equivalent to . Also, a-cD, which is equivalent to , 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.) ()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.