Regular Expressions 101

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/
^((^|xx)(^|\3\4\4)(^|\4x)(^x|\1))*$
/
gm
^ asserts position at start of a line
1st Capturing Group
((^|xx)(^|\3\4\4)(^|\4x)(^x|\1))*
* 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
2nd Capturing Group
(^|xx)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
xx
xx matches the characters xx literally (case sensitive)
3rd Capturing Group
(^|\3\4\4)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
\3\4\4
\3 matches the same text as most recently matched by the 3rd capturing group
\4 matches the same text as most recently matched by the 4th capturing group
\4 matches the same text as most recently matched by the 4th capturing group
4th Capturing Group
(^|\4x)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
\4x
\4 matches the same text as most recently matched by the 4th capturing group
x
matches the character x literally (case sensitive)
5th Capturing Group
(^x|\1)
1st Alternative
^x
^ asserts position at start of a line
x matches the character x literally (case sensitive)
2nd Alternative
\1
$ 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