add PIRegularExpression

3rd/pcre2/testdata/testinput23

@@ -0,0 +1,9 @@
+# This test is run when PCRE2 has been built with --enable-never-backslash-C,
+# which disables the use of \C. All we can do is check that it gives the 
+# correct error message.
+
+/a\Cb/
+
+/a[\C]b/
+
+# End of testinput23