Regular Expressions

Regular expressions are a simple pattern matching language used for locating text in a file. All regular expressions are constructed from series of one or more single character expressions. Single character expressions can take several forms:

Typing chars	A-Z a-z 0-9 ! @ # %	Any alphanumeric or symbol char that
	< > ( ) { } , ~ | :	can be typed except chars used in
	; ? + = - _ <tab>	substitution. These chars match only
	<blk> <ctrl chars>	identical chars in text. We must precede
		? with \ if ? is used as first
		char in a backward search.
Substitution	. ^ $ / [ ] \ * -	These chars represent another char or
or search		beg or end of line or serve as
control chars		delimiters, range identifiers, or escape
		chars in regular expressions. However,
		under certain conditions, - and ]
		are interpreted directly as explained
		below.
Sets or ranges	[set_of_chars] or	A group of single chars or range of
of chars	[range_of_chars] or	chars enclosed within a pair of square brackets [ ]
	[combination_of_both]	(such as [actz58&] or [3-7]) where a
		match is accepted if any of the chars
		between the [] or in the specified
		range appears in the position defined by
		the position of the single char
		expression in a larger expression. The
		second form example accepts a match if
		3, 4, 5, 6 or 7 appears in the position
		indicated.

The - is interpreted as a range specifier when defining sets of characters, as in the single character expression [a-z], unless it is the first character in a set of characters, as in the expression [-abdfgh12], which match any one of the characters -, a , b, d, f, h, 1, or 2. Likewise, the ] terminates the expression unless it is the first character in the set, as in the group []=+rt12], which matches any one of the characters ], =, +, r, t, 1 or 2.