Regular Expressions 101

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/
^((?(1)((?(2)\2((?(3)\3((?(4)\4x|x))|x))|x))|x))*$
/
gm
^ asserts position at start of a line
1st Capturing Group
((?(1)((?(2)\2((?(3)\3((?(4)\4x|x))|x))|x))|x))*
* 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
Conditional
(?(1)((?(2)\2((?(3)\3((?(4)\4x|x))|x))|x))|x)
Conditionally matches one of two options depending on whether the 1st capturing group matched
If condition is met, match the following regex
((?(2)\2((?(3)\3((?(4)\4x|x))|x))|x))
2nd Capturing Group
((?(2)\2((?(3)\3((?(4)\4x|x))|x))|x))
Conditional
(?(2)\2((?(3)\3((?(4)\4x|x))|x))|x)
Else match the following regex
x
x matches the character x literally (case sensitive)
$ 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