Description

before(expr) succeeds if the expr sub-expression would match at the current position in the sequence, but expr is not included in the match. For instance, before("foo") succeeds if we are before a "foo". Look-ahead assertions can be negated with the bit-compliment operator.

	Note
	before(expr) is equivalent to the perl (?=...) extension. ~before(expr) is a negative look-ahead assertion, equivalent to the perl (?!...) extension.