Regular Expressions 101

Non-Capturing Group(?:^(?=\s*\S+(?:\s+\S+){0,3}\s*$)|\G(?!^))

1st Alternative^(?=\s*\S+(?:\s+\S+){0,3}\s*$)

^start of string

Positive Lookahead(?=\s*\S+(?:\s+\S+){0,3}\s*$)

\s*any whitespace character, repeated any number of times

\S+any non-whitespace character, repeated at least once

Non-Capturing Group(?:\s+\S+){0,3}

{0,3}repeats the group contents below between 0 and 3 times:

\s+any whitespace character, repeated at least once

\S+any non-whitespace character, repeated at least once

\s*any whitespace character, repeated any number of times

$end of string, or before its final line terminator

2nd Alternative\G(?!^)

\Gstart of current search

Negative Lookahead(?!^)

^start of string

\s*any whitespace character, repeated any number of times

\Kresets the starting point of the reported match. Any previously consumed characters are no longer included in the final match

\S+any non-whitespace character, repeated at least once

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

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Search reference

Regular Expression

(?:^(?=\s*\S+(?:\s+\S+){0,3}\s*$)|\G(?!^))\s*\K\S+

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