The pattern matching facilities have serious restrictions in that they will not work properly with sections, and they cannot be used with lexically scoped variables, which is why all pattern variables have to be declared with "vars", not "lvars". (See HELP LVARS).
There is a modified version of the matcher described in HELP FMATCHES which overcomes these restrictions, though it is no longer an infix operator, but a syntax word.
FMATCHES is superseded by the DOESMATCH library and the READPATTERN library, both available from the Birmingham FTP directory.
In a later version of Pop-11 both may be superseded.