Regular Expressions 101

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"
\[(?:[a-zA-Z0-9]{0,4}:?){1,8}\]|\[(?:[a-zA-Z0-9]{0,4}:?){1,4}(?:[0-9]{1,3}\.){3}[0-9]{1,3}\]
"
g
1st Alternative
\[(?:[a-zA-Z0-9]{0,4}:?){1,8}\]
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
Non-capturing group
(?:[a-zA-Z0-9]{0,4}:?){1,8}
{1,8} matches the previous token between 1 and 8 times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-zA-Z0-9]
{0,4} matches the previous token between 0 and 4 times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
:
matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
2nd Alternative
\[(?:[a-zA-Z0-9]{0,4}:?){1,4}(?:[0-9]{1,3}\.){3}[0-9]{1,3}\]
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
Non-capturing group
(?:[a-zA-Z0-9]{0,4}:?){1,4}
{1,4} matches the previous token between 1 and 4 times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-zA-Z0-9]
{0,4} matches the previous token between 0 and 4 times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
:
matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
Non-capturing group
(?:[0-9]{1,3}\.){3}
{3} matches the previous token exactly 3 times
Match a single character present in the list below
[0-9]
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[0-9]
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
Global pattern flags
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"
"
g

Test String