Regular Expressions 101

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/
(?(DEFINE) (?P<list>(?:apple|banana|orange);) ) ^(?!(?P>list)*?((?P>list))(?P>list)*?\2) (?P>list)+$
/
gmx
Subpattern definition construct
(?(DEFINE) (?P<list>(?:apple|banana|orange);) )
May only be used to define functions. No matching is done in this group.
Named Capture Group list
(?P<list>(?:apple|banana|orange);)
Non-capturing group
(?:apple|banana|orange)
1st Alternative
apple
apple
matches the characters apple literally (case sensitive)
2nd Alternative
banana
banana
matches the characters banana literally (case sensitive)
3rd Alternative
orange
orange
matches the characters orange literally (case sensitive)
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
^ asserts position at start of a line
Negative Lookahead
(?!(?P>list)*?((?P>list))(?P>list)*?\2)
Assert that the Regex below does not match
(?P>list)
recurses the subpattern named list
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
2nd Capturing Group
((?P>list))
(?P>list) recurses the subpattern named list
(?P>list)
recurses the subpattern named list
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
\2 matches the same text as most recently matched by the 2nd capturing group
(?P>list)
recurses the subpattern named list
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
$ 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)
x modifier: extended. Spaces and text after a # in the pattern are ignored
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gmx

Test String