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
- \d
- [[:digit:]]
- \s
- [[:space:]]
- \w
- [[:alnum:]_] (note underscore)
- \D
- [^[:digit:]]
- \S
- [^[:space:]]
- \W
- [^[:alnum:]_] (note underscore)
- \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
