IEEE Standards Interpretations for IEEE Std 1003.2™-1992 IEEE Standard for Information Technology--Portable Operating System Interfaces (POSIX®)--Part 2: Shell and Utilities
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Interpretation Request #82
Topic: BRE Relevant Clauses: 2.8.3
POSIX.2 Subclause 18.104.22.168 item (3) specifies the semantics of backreference expressions in Basic Regular Expressions. This subclause states in part: The backreference expression \n shall match the same (possibly empty) string of characters as was matched by a subexpression enclosed between \( and \) preceding the \n. Suppose that the subexpression includes an anchor, and suppose that the system in question is one on which anchoring characters act as anchors in subexpressions. Does the backreference expression have to match an anchored string? (In this case, it can never match anything unless the first subexpression matched an empty string.) For example, on such a system does the BRE \(^b\)\1 match the first two characters of bbbbb or is the BRE to be treated as equivalent to \(^b\)\(^b\) (in which case it will not match)?
The standard clearly states the behaviour for BREs, and conforming implementations must conform to this. In the example provided, the standard permits matching, but does not require it.
Rationale for Interpretation