Interpretations

Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

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 #73
Topic: extended regular expressions Relevant Clauses: 2.8.4

Subclause 2.8.4 of POSIX.2 specifies the syntax and semantics of Extended Regular Expressions. Subclause 2.8.4.6 describes the effects of anchoring EREs, including ERE subexpressions. The language of this subclause seems to clearly indicate that within ERE subexpressions but outside of bracket expressions the caret and the dollar sign are always treated as anchors. The issue on which I would like an interpretation has to do with nested anchored subexpressions. Are such subexpressions valid? For example, is the ERE ^(^(^a$($($ a valid expression, and does it match a single line containing only the character “a”?

Interpretation Response
The rules specified in 2.4.8.3 item 1 and 2.8.4.6 clearly state that this is a valid ERE and as suggested shall match the character “a”. Conforming implementations must conform to this.

Rationale for Interpretation
None.