Regular Expressions 101

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"
(to|from)=?<([a-zA-Z0-9_.-]+)\@([A-Za-z_-]+)\.(([A-Za-z]{3})|([A-Za-z]{3})\.([A-Za-z]{2}?))>
"
gm
1st Capturing Group
(to|from)
1st Alternative
to
to
matches the characters to literally (case sensitive)
2nd Alternative
from
from
matches the characters from literally (case sensitive)
=
matches the character = with index 6110 (3D16 or 758) literally (case sensitive)
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
2nd Capturing Group
([a-zA-Z0-9_.-]+)
Match a single character present in the list below
[a-zA-Z0-9_.-]
+ matches the previous token between one and unlimited 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)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
_.-
matches a single character in the list _.- (case sensitive)
\@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
3rd Capturing Group
([A-Za-z_-]+)
Match a single character present in the list below
[A-Za-z_-]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
_-
matches a single character in the list _- (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
4th Capturing Group
(([A-Za-z]{3})|([A-Za-z]{3})\.([A-Za-z]{2}?))
1st Alternative
([A-Za-z]{3})
5th Capturing Group
([A-Za-z]{3})
Match a single character present in the list below
[A-Za-z]
{3} matches the previous token exactly 3 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
2nd Alternative
([A-Za-z]{3})\.([A-Za-z]{2}?)
6th Capturing Group
([A-Za-z]{3})
Match a single character present in the list below
[A-Za-z]
{3} matches the previous token exactly 3 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
7th Capturing Group
([A-Za-z]{2}?)
Match a single character present in the list below
[A-Za-z]
{2}? matches the previous token exactly 2 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
> matches the character > with index 6210 (3E16 or 768) 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