Regular Expressions 101

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/
^(\S++).*\K(?:\R\1(?:\h.*|$))+
/
gm
^ asserts position at start of a line
1st Capturing Group
(\S++)
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
++ matches the previous token between one and unlimited times, as many times as possible, without giving back (possessive)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\K resets the starting point of the reported match. Any previously consumed characters are no longer included in the final match
Non-capturing group
(?:\R\1(?:\h.*|$))+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\R matches any Unicode newline sequence
\1 matches the same text as most recently matched by the 1st capturing group
Non-capturing group
(?:\h.*|$)
1st Alternative
\h.*
\h matches any horizontal whitespace character (equivalent to [[:blank:]])
.
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)
2nd Alternative
$
$ asserts position at the end of a line
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

Substitution

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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