Regular Expressions 101

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/
^From:\s[A-Za-z]+\s[A-Za-z]+(\s[A-Za-z]+)?(\s[A-Za-z]+)?\s<[A-Za-z]+@domain\.com>
/
gm
^ asserts position at start of a line
From:
matches the characters From: literally (case sensitive)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
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)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
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)
1st Capturing Group
(\s[A-Za-z]+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
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)
2nd Capturing Group
(\s[A-Za-z]+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
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)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
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)
@domain
matches the characters @domain literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
com>
matches the characters com> 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

Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for Python, please visit: https://docs.python.org/3/library/re.html