Regular Expressions 101

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/
^(?:(\S+)|\G) (?=.*?(\1[^\n]+))
/
gms
^ asserts position at start of a line
Non-capturing group
(?:(\S+)|\G)
1st Alternative
(\S+)
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)
2nd Alternative
\G
\G asserts position at the end of the previous match or the start of the string for the first match
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Positive Lookahead
(?=.*?(\1[^\n]+))
Assert that the Regex below matches
.
matches any character
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
2nd Capturing Group
(\1[^\n]+)
\1 matches the same text as most recently matched by the 1st capturing group
Match a single character not present in the list below
[^\n]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\n matches a line-feed (newline) character (ASCII 10)
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)
s modifier: single line. Dot matches newline characters
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gms

Test String

Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php