Regular Expressions 101

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/
<p(?:(?!<|>).)*>((?:(?!<p|<\/p>).|\s)*)<\/p>
/
gm
<p
matches the characters <p literally (case sensitive)
Non-capturing group
(?:(?!<|>).)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?!<|>)
Assert that the Regex below does not match
1st Alternative
<
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
2nd Alternative
>
> matches the character > with index 6210 (3E16 or 768) literally (case sensitive)
. matches any character (except for line terminators)
> matches the character > with index 6210 (3E16 or 768) literally (case sensitive)
1st Capturing Group
((?:(?!<p|<\/p>).|\s)*)
Non-capturing group
(?:(?!<p|<\/p>).|\s)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
1st Alternative
(?!<p|<\/p>).
Negative Lookahead
(?!<p|<\/p>)
Assert that the Regex below does not match
1st Alternative
<p
<p
matches the characters <p literally (case sensitive)
2nd Alternative
<\/p>
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
p>
matches the characters p> literally (case sensitive)
. matches any character (except for line terminators)
2nd Alternative
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
p>
matches the characters p> 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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String