Regular expressions are built from the following abstract syntax:

regexp	::=	unionexp
unionexp	::=	interexp | unionexp	(union)
	|	interexp
interexp	::=	concatexp & interexp	(intersection)	[OPTIONAL]
	|	concatexp
concatexp	::=	repeatexp concatexp	(concatenation)
	|	repeatexp
repeatexp	::=	repeatexp ?	(zero or one occurrence)
	|	repeatexp *	(zero or more occurrences)
	|	repeatexp +	(one or more occurrences)
	|	repeatexp {n}	(n occurrences)
	|	repeatexp {n,}	(n or more occurrences)
	|	repeatexp {n,m}	(n to m occurrences, including both)
	|	complexp
complexp	::=	~ complexp	(complement)	[OPTIONAL]
	|	charclassexp
charclassexp	::=	[ charclasses ]	(character class)
	|	[^ charclasses ]	(negated character class)
	|	simpleexp
charclasses	::=	charclass charclasses
	|	charclass
charclass	::=	charexp - charexp	(character range, including end-points)
	|	charexp
simpleexp	::=	charexp
	|	.	(any single character)
	|	#	(the empty language)	[OPTIONAL]
	|	@	(any string)	[OPTIONAL]
	|	" <Unicode string without double-quotes> "	(a string)
	|	( )	(the empty string)
	|	( unionexp )	(precedence override)
	|	< <identifier> >	(named automaton)	[OPTIONAL]
	|	<n-m>	(numerical interval)	[OPTIONAL]
charexp	::=	<Unicode character>	(a single non-reserved character)
	|	\ <Unicode character>	(a single character)

The productions marked [OPTIONAL] are only allowed if specified by the syntax flags passed to the RegExp constructor. The reserved characters used in the (enabled) syntax must be escaped with backslash (\) or double-quotes ("..."). (In contrast to other regexp syntaxes, this is required also in character classes.) Be aware that dash (-) has a special meaning in charclass expressions. An identifier is a string not containing right angle bracket (>) or dash (-). Numerical intervals are specified by non-negative decimal integers and include both end points, and if n and m have the same number of digits, then the conforming strings must have that length (i.e. prefixed by 0's). @author Anders Møller <amoeller@brics.dk>