Regular Expressions 101

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/
(\b[\pL0-9]++\b)(?=.*?\b\1\b)
/
gm
1st Capturing Group
(\b[\pL0-9]++\b)
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Match a single character present in the list below
[\pL0-9]
++ matches the previous token between one and unlimited times, as many times as possible, without giving back (possessive)
\pL matches any kind of letter from any language
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Positive Lookahead
(?=.*?\b\1\b)
Assert that the Regex below matches
.
matches any character (except for line terminators)
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
\1 matches the same text as most recently matched by the 1st capturing group
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
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