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B.6 Grouping Constructs

The following table describes the grouping constructs. Grouping constructs allow you to capture groups of sub-expressions and to increase the efficiency of regular expressions with non-capturing look ahead and look behind modifiers.

Table B.7

List of Grouping Constructs

Grouping Constructs

Description

( )

Captures the matched substring if used in a find and replace operation.

(?=)

Zero-width positive look ahead assertion. Continues match only if the sub-expression matches at this position on the right. For example, (_?=\w) matches an underscore followed by a word character without matching the word character.

(?!)

Zero-width negative look ahead assertion. Continues match only if the sub-expression matches at this position on the right. For example, \b(?!un)\w+\b matches words that do not begin with un.

(?<=)

Zero-width positive look behind assertion. Continues match only if the sub-expression matches the position on the left. For example, (?<=19)99 matches instances of 99 that follow 19.

(?<!)

Zero width negative look behind assertion. Continues match only if the sub-expression does not match this position on the left.