Regular Expressions 101

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"
^127\.\d{1,3}\.\d{1,3}\.\d{1,3}(\:\d+)?$
"
mg
^ asserts position at start of a line
127
matches the characters 127 literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
(\:\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
+ 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 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)
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"
"
mg

Test String