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.

--- a/NEWS
+++ b/NEWS
@@ -12,6 +12,13 @@
     octal: '\oNNN' or '\o{NNN}'
     hex  : '\xNN'  or '\x{NN}'
 
+ ** Unknown escape sequences in the replacement string for regexprep are now
+    substituted with their unescaped version and no warning is emitted.
+    This change was made for Matlab compatibility.
+
+    Example: regexprep ('a', 'a', 'x\yz')
+             => 'xyz'
+
  ** mkfifo now interprets the MODE argument as an octal, not decimal, integer.
     This is consistent with the equivalent shell command.