Regular Expressions 101

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/
^(?!.*?(?:^|;)((?:apple|banana|orange))(?=;|$).*;\1(?:;|$))(?:(?:apple|banana|orange);)+$
/
gm
^ asserts position at start of a line
Negative Lookahead
(?!.*?(?:^|;)((?:apple|banana|orange))(?=;|$).*;\1(?:;|$))
Assert that the Regex below does not match
.
matches any character (except for line terminators)
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
Non-capturing group
(?:^|;)
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
;
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
1st Capturing Group
((?: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)
Positive Lookahead
(?=;|$)
Assert that the Regex below matches
1st Alternative
;
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
2nd Alternative
$
$ asserts position at the end of a line
.
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)
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
\1 matches the same text as most recently matched by the 1st capturing group
Non-capturing group
(?:;|$)
1st Alternative
;
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
2nd Alternative
$
$ asserts position at the end of a line
Non-capturing group
(?:(?:apple|banana|orange);)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
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 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