APPENDIX C Regular Expressions
The POSIX specification defines two kinds of regular expressions: basic regular expressions, such as those used by historic
grep, and extended regular expressions, used by
egrep). There are other kinds, but these two form the basis of what you need to know. While some documentation describes basic regular expressions as "obsolete," they are still in widespread use. (However, if you are developing software, please use extended regular expressions.)
This appendix is also significant because the shell pattern matching function (see Appendix A for the complete shell specification) is defined in terms of section 9.3.5, "RE Bracket Expression."
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