One of the options I mentioned in a prior email about getting case-insensitive REs in ed was to support a more sophisticated flavor of regular expression. This is the fruit of that idea.
The attached patch (Made against 1.18-pre2) adds a -P option to use PCRE2 regular expressions. Passing --disable-pcre2 to the configure script will leave this feature out. There's also a --utf8 option that turns on PCRE2's advanced Unicode matching. It also adds some tests for ed -E and ed -P, and mentions both in the regular expression section of the texi documentation.
pcre2.patch
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