regexp-replace and ^, word boundary or look-behind operators

[Stephane Chazelas <stephane.chazelas@xxxxxxxxx>]

The way regexp-replace works means that these things:

$ a='aaab'; regexp-replace a '^a' x; echo "$a"
xxxb
$ a='abab'; regexp-replace a '\<ab' '<$MATCH>'; echo $a
<ab><ab>
$ set -o rematchpcre
$ a=xxx; regexp-replace a '(?<!x)x' y; echo $a
yyy

don't work properly as after the first substitution, the regex
is no longer matched on the full subject, but on the part of
subject after the last match.

I don't think that can be fixed without exposing more of the
regex/pcre C API.

-- 
Stephane