Regular Expressions 101

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/
\b(\w+?)\1\b
/
g
\b assert position at a word boundary: (^\\w|\\w$|\\W\\w|\\w\\W)
1st Capturing Group
(\w+?)
\w+?
matches any word character (equal to [a-zA-Z0-9_])
+? Quantifier — Matches between one and unlimited times, as few times as possible, expanding as needed (lazy)
\1 matches the same text as most recently matched by the 1st capturing group
\b assert position at a word boundary: (^\\w|\\w$|\\W\\w|\\w\\W)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String