Regular Expressions 101

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/
([A-Z])\1
/
gm
1st Capturing Group
([A-Z])
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\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)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String