Regular Expressions 101

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"
(?=^.{4,253}$)(^((?!-)[a-zA-Z0-9-]{1,63}(?<!-)\.)+[a-zA-Z]{2,63}$)
"
Positive Lookahead
(?=^.{4,253}$)
Assert that the Regex below matches
^ asserts position at start of the string
.
matches any character (except for line terminators)
{4,253} matches the previous token between 4 and 253 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)
1st Capturing Group
(^((?!-)[a-zA-Z0-9-]{1,63}(?<!-)\.)+[a-zA-Z]{2,63}$)
^ asserts position at start of the string
2nd Capturing Group
((?!-)[a-zA-Z0-9-]{1,63}(?<!-)\.)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Negative Lookahead
(?!-)
Assert that the Regex below does not match
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
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 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 4510 (2D16 or 558) literally (case sensitive)
Negative Lookbehind
(?<!-)
Assert that the Regex below does not match
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[a-zA-Z]
{2,63} matches the previous token between 2 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 sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
$ 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.

Regular Expression
No Match

r"
"

Test String