Regular Expressions 101

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/
\.(?!html?)([a-z]{0,3}|[a-z]{0,4})$
/
gm
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Negative Lookahead
(?!html?)
Assert that the Regex below does not match
htm
matches the characters htm literally (case sensitive)
l
matches the character l with index 10810 (6C16 or 1548) literally (case sensitive)
1st Capturing Group
([a-z]{0,3}|[a-z]{0,4})
1st Alternative
[a-z]{0,3}
Match a single character present in the list below
[a-z]
{0,3} matches the previous token between 0 and 3 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)
2nd Alternative
[a-z]{0,4}
Match a single character present in the list below
[a-z]
{0,4} matches the previous token between 0 and 4 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)
$ 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.

Regular Expression
No Match

/
/
gm

Test String