"\(ac*\)c*d*\1" against acdacaaa matches acdacaaa (with \1= a) simply matching the longest matchįor "\(ac*\)" would yield \1= ac, but the overall match would be smaller ( acdac). Subexpression, but only with the proviso that the overall match is leftmost longest. It is possible to determine what strings correspond to subexpressions by recursively applying the leftmost longest rule to each The historical RE syntax and rules to accommodate other languages.Īs they are limited to bracket expressions, the rationale for these modifications is in XBD RE Bracket Expression.
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It was necessary to provide extensions to Not provide the necessary support for commonly used characters and language constructs. The historical rules and behavior do not provide that capability to non-English language users, and do The syntax and rules of REs are intended to be as intuitive as possible to make themĮasy to understand and use. Operations in business and other operations. The specification of REs is particularly important to internationalization because pattern matching operations are very basic It should be noted that the definitions in this section do not cover the tr utility The text is based on the ed description and substantially modified, primarily to aidĭevelopers and others in the understanding of the capabilities and limitations of REs. The BRE corresponds to the ed or historical grep type, and the ERE corresponds to the historical egrep type (now grep -E). The most common behavior is described here, and exceptions orĮxtensions to this are documented for the respective utilities, as appropriate.
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Rather than repeating the description of REs for each utility supporting REs, the standard developers preferred a common,Ĭomprehensive description of regular expressions in one place. The Open Group Base Specifications Isedition