Process backslashes in regexprep replacement strings in Matlab compatible fashion (bug #45407). * NEWS: Announce change. Backslashed escaped ordinary characters are now replaced by the character itself with no emitted warning. * regexp.cc (do_regexp_rep_string_escapes): Change default case statement to silently convert unrecognized escaped character to the bare character itself. Pass the special sequences of '$' and '\' through since these must be handled by lo-regexp.cc. * regexp.cc (Fregexprep): Add BIST tests for new behavior.

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