Regular Expressions 101

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"
^.*IMAP Error: Login failed for .* from (?P<host>.*)\. AUTHENTICATE PLAIN: Authentication failed\. in .* \(POST .*\)$
"
gm
^ asserts position at start of a line
.
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)
IMAP Error: Login failed for
matches the characters IMAP Error: Login failed for literally (case sensitive)
.
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)
from
matches the characters from literally (case sensitive)
Named Capture Group host
(?P<host>.*)
.
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 4610 (2E16 or 568) literally (case sensitive)
AUTHENTICATE PLAIN: Authentication failed
matches the characters AUTHENTICATE PLAIN: Authentication failed literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
in
matches the characters in literally (case sensitive)
.
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)
\( matches the character ( with index 4010 (2816 or 508) literally (case sensitive)
POST
matches the characters POST literally (case sensitive)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\) matches the character ) with index 4110 (2916 or 518) literally (case sensitive)
$ 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