Regular Expressions 101

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/
^(\w+) =.*\n(?=((?!\b\1\b)[^!])*(^\1 =|\Z))
/
gm
^ asserts position at start of a line
1st Capturing Group
(\w+)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
=
matches the characters = literally (case sensitive)
.
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)
\n matches a line-feed (newline) character (ASCII 10)
Positive Lookahead
(?=((?!\b\1\b)[^!])*(^\1 =|\Z))
Assert that the Regex below matches
2nd Capturing Group
((?!\b\1\b)[^!])*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Negative Lookahead
(?!\b\1\b)
Assert that the Regex below does not match
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
\1 matches the same text as most recently matched by the 1st capturing group
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Match a single character not present in the list below
[^!]
! matches the character ! with index 3310 (2116 or 418) literally (case sensitive)
3rd Capturing Group
(^\1 =|\Z)
1st Alternative
^\1 =
^ asserts position at start of a line
\1 matches the same text as most recently matched by the 1st capturing group
=
matches the characters = literally (case sensitive)
2nd Alternative
\Z
\Z asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
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

Processing...