Regular Expressions 101

All the errors detected are listed below, from left to right, as they appear in the pattern.

\1 Python backreference target must be defined and closed earlier

\2 Python backreference target must be defined and closed earlier

\3 Python backreference target must be defined and closed earlier

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

^(?:(?:(?!\1)()|\1 )(?:(?!\2)()fat|(?!\3)()cat)){1,2}$

New

fat cat