Regular Expressions 101

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/
(?<foo>[abc])\k<foo>
/
g
Named Capture Group foo
(?<foo>[abc])
Match a single character present in the list below
[abc]
abc matches a single character in the list abc (case sensitive)
\k<foo> matches the same text as most recently matched by the capturing group named foo
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String