Regular Expressions 101

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/
^(\S+) (?=(?s:.)*\1.*).*
/
gm
^ asserts position at start of a line
1st Capturing Group
(\S+)
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Positive Lookahead
(?=(?s:.)*\1.*)
Assert that the Regex below matches
Non-capturing Group. Matches the tokens contained with the following effective flags: gms
(?s:.)*
s modifier: single line. Dot matches newline characters
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
. matches any character
\1 matches the same text as most recently matched by the 1st capturing group
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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

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