Regular Expressions 101

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/
.*Disconnected from (invalid |authenticating )?user .* [^ ]+ port [0-9]+( \[preauth\])?$
/
gm
.
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)
Disconnected from
matches the characters Disconnected from literally (case sensitive)
1st Capturing Group
(invalid |authenticating )?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
invalid
invalid
matches the characters invalid literally (case sensitive)
2nd Alternative
authenticating
authenticating
matches the characters authenticating literally (case sensitive)
user
matches the characters user 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)
Match a single character not present in the list below
[^ ]
+ matches the previous token between one 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)
port
matches the characters port literally (case sensitive)
Match a single character present in the list below
[0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Capturing Group
( \[preauth\])?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
preauth
matches the characters preauth literally (case sensitive)
\] matches the character ] with index 9310 (5D16 or 1358) 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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String