Regular Expressions 101

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/
<(?'Priority'[0-9]{1,3})>(?'Date'[a-zA-Z]{1,3}\s(?(?=\s)\s|[0-9])[0-9])\s(?'Time'(?:[0-1][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9])\s(?'Host'(?(?=\d)(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+)|\w+)\s(?'Message'.+$)
/
< matches the character < with index 6010 (3C16 or 748) literally (case sensitive)
Named Capture Group Priority
(?'Priority'[0-9]{1,3})
Match a single character present in the list below
[0-9]
{1,3} matches the previous token between 1 and 3 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)
> matches the character > with index 6210 (3E16 or 768) literally (case sensitive)
Named Capture Group Date
(?'Date'[a-zA-Z]{1,3}\s(?(?=\s)\s|[0-9])[0-9])
Match a single character present in the list below
[a-zA-Z]
{1,3} matches the previous token between 1 and 3 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)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Conditional
(?(?=\s)\s|[0-9])
Conditionally matches one of two options depending on whether the lookaround matches
Positive Lookahead
(?=\s)
Assert that the Regex below matches
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
If condition is met, match the following regex
\s
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Else match the following regex
[0-9]
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Named Capture Group Time
(?'Time'(?:[0-1][0-9]|2[0-3]):[0-5][0-9]:[0-5][0-9])
Non-capturing group
(?:[0-1][0-9]|2[0-3])
1st Alternative
[0-1][0-9]
Match a single character present in the list below
[0-1]
0-1 matches a single character in the range between 0 (index 48) and 1 (index 49) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Alternative
2[0-3]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-3]
0-3 matches a single character in the range between 0 (index 48) and 3 (index 51) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Named Capture Group Host
(?'Host'(?(?=\d)(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+)|\w+)
1st Alternative
(?(?=\d)(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+)
Conditional
(?(?=\d)(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+)
Conditionally matches one of two options depending on whether the lookaround matches
Positive Lookahead
(?=\d)
Assert that the Regex below matches
\d matches a digit (equivalent to [0-9])
If condition is met, match the following regex
(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+
Non-capturing group
(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}
{3} matches the previous token exactly 3 times
Non-capturing group
(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
1st Alternative
25[0-5]
25
matches the characters 25 literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
2nd Alternative
2[0-4][0-9]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-4]
0-4 matches a single character in the range between 0 (index 48) and 4 (index 52) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Alternative
[01]?[0-9][0-9]?
Match a single character present in the list below
[01]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
01
matches a single character in the list 01 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
? matches the previous token between zero and one 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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Non-capturing group
(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
1st Alternative
25[0-5]
25
matches the characters 25 literally (case sensitive)
Match a single character present in the list below
[0-5]
0-5 matches a single character in the range between 0 (index 48) and 5 (index 53) (case sensitive)
2nd Alternative
2[0-4][0-9]
2 matches the character 2 with index 5010 (3216 or 628) literally (case sensitive)
Match a single character present in the list below
[0-4]
0-4 matches a single character in the range between 0 (index 48) and 4 (index 52) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Alternative
[01]?[0-9][0-9]?
Match a single character present in the list below
[01]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
01
matches a single character in the list 01 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
? matches the previous token between zero and one 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 Alternative
\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)
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
Named Capture Group Message
(?'Message'.+$)
.
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)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String