Regular Expressions 101

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"
(?<=<p>)(.*) (?=</p>)
"
gm
Positive Lookbehind
(?<=<p>)
Assert that the Regex below matches
<p>
matches the characters <p> literally (case sensitive)
1st Capturing Group
(.*)
.
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 3210 (2016 or 408) literally (case sensitive)
Positive Lookahead
(?=</p>)
Assert that the Regex below matches
</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.

Regular Expression
No Match

r"
"
gm

Test String