Regular Expressions 101

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"
^(.+)(\@)(\w+)(\.)(\w{3,4})$
"
gm
^ asserts position at start of a line
1st Capturing Group
(.+)
.
matches any character (except for line terminators)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
(\@)
\@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
3rd Capturing Group
(\w+)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
4th Capturing Group
(\.)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
5th Capturing Group
(\w{3,4})
\w
matches any word character (equivalent to [a-zA-Z0-9_])
{3,4} matches the previous token between 3 and 4 times, as many times as possible, giving back as needed (greedy)
$ 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

r"
"
gm

Test String