## Escapes

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:

 d s w (note underscore) D S W (note underscore)

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.

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