Regular Expressions 101

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"
&(?!#?[a-z0-9]+;)
"
gm
& matches the character & with index 3810 (2616 or 468) literally (case sensitive)
Negative Lookahead
(?!#?[a-z0-9]+;)
Assert that the Regex below does not match
#
matches the character # with index 3510 (2316 or 438) literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
; matches the character ; with index 5910 (3B16 or 738) literally (case sensitive)
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.

Regular Expression
No Match

r"
"
gm

Test String