Regular Expressions 101

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"
^([a-zA-Z0-9-_]{0,63}\.?[a-zA-Z0-9-_]{1,63}\.[a-zA-Z0-9-_]{1,63}):?(\d{1,5})?$
"
mig
^ asserts position at start of a line
1st Capturing Group
([a-zA-Z0-9-_]{0,63}\.?[a-zA-Z0-9-_]{1,63}\.[a-zA-Z0-9-_]{1,63})
Match a single character present in the list below
[a-zA-Z0-9-_]
{0,63} matches the previous token between 0 and 63 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 insensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case insensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case insensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
_ matches the character _ with index 9510 (5F16 or 1378) literally (case insensitive)
\.
matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-zA-Z0-9-_]
{1,63} matches the previous token between 1 and 63 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 insensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case insensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case insensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
_ matches the character _ with index 9510 (5F16 or 1378) literally (case insensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
Match a single character present in the list below
[a-zA-Z0-9-_]
{1,63} matches the previous token between 1 and 63 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 insensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case insensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case insensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
_ matches the character _ with index 9510 (5F16 or 1378) literally (case insensitive)
:
matches the character : with index 5810 (3A16 or 728) literally (case insensitive)
2nd Capturing Group
(\d{1,5})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\d
matches a digit (equivalent to [0-9])
{1,5} matches the previous token between 1 and 5 times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of a line
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
mig

Test String