In Regexes§

See primary documentation in context for Ignoremark

The :ignoremark or :m adverb instructs the regex engine to only compare base characters, and ignore additional marks such as combining accents:

say so 'a' ~~ rx/ä/;                # OUTPUT: «False␤» 
say so 'a' ~~ rx:ignoremark /ä/;    # OUTPUT: «True␤» 
say so '' ~~ rx:ignoremark /o/;    # OUTPUT: «True␤»