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 #137
Topic: Regular Expressions Relevant Sections: 126.96.36.199
Section 188.8.131.52 says: 1785 The ex utility shall support the BREs described in 2.8.3. A null BRE ((//) shall be 1786 equivalent to the last BRE encountered. Historically, however, ex/vi regular expressions have differed from the BRE's described in .8.3 in one respect - they accepted backslash as quoting the following characters within brackets: \, -, ^, and ]. Therefore, the pattern [\^abc] would not historically match \, but only ^, a, b or c. Was it the intention to change ex/vi's regular expressions in this way?
The standard clearly states the requirements for BREs in ex and conforming implementations must conform to this. It is true that historical versions of many utilities had minor varia$$$nts in regular expressions, POSIX.2 regularized most of these utilities including all of the editors to use the standard regular expressions supported by the regular expression standard routines.
Rationale for Interpretation