Regular Expressions 101

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/
^(?!(x(?=(x$)((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).)))|$))
/
gm
^ asserts position at start of a line
Negative Lookahead
(?!(x(?=(x$)((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).)))|$))
Assert that the Regex below does not match
1st Capturing Group
(x(?=(x$)((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).)))|$)
1st Alternative
x(?=(x$)((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).)))
x matches the character x literally (case sensitive)
Positive Lookahead
(?=(x$)((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).)))
Assert that the Regex below matches
2nd Capturing Group
(x$)
x
matches the character x literally (case sensitive)
$ asserts position at the end of a line
3rd Capturing Group
((?<=(?=^(?(?=\2$)(?=(\4\2))).(?=\2$)(?1)|(?3)).))
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