Regular Expressions 101

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/
([a-z]+)\1
/
gi
1st Capturing Group
([a-z]+)
Match a single character present in the list below
[a-z]+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z a single character in the range between a (index 97) and z (index 122) (case insensitive)
\1 matches the same text as most recently matched by the 1st capturing group
Global pattern flags
g modifier: global. All matches (don't return after first match)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gi

Test String