Regular Expressions 101

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  • PCRE2 (PHP >=7.3)
  • PCRE (PHP <7.3)
  • ECMAScript (JavaScript)
  • Python 2.7
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/
\.(?!html?)([a-z]{0,3}|[a-z]{0,4})$
/
gm
\. matches the character . 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 literally (case sensitive)
? Quantifier — Matches between zero and one times, as many times as possible, giving back as needed (greedy)
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}
{0,3} Quantifier — Matches between 0 and 3 times, as many times as possible, giving back as needed (greedy)
a-z 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}
{0,4} Quantifier — Matches between 0 and 4 times, as many times as possible, giving back as needed (greedy)
a-z 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