Regular Expressions 101

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`
(?i)^(?:([a-z0-9-]+|\*)\.)?([a-z0-9-]{1,61})\.([a-z0-9]{2,7})$
`
gm
(?i)
match the remainder of the pattern with the following effective flags: gmi
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
^ asserts position at start of a line
Non-capturing group
(?:([a-z0-9-]+|\*)\.)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Capturing Group
([a-z0-9-]+|\*)
1st Alternative
[a-z0-9-]+
Match a single character present in the list below
[a-z0-9-]
+ matches the previous token between one and unlimited 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)
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)
2nd Alternative
\*
\* matches the character * with index 4210 (2A16 or 528) literally (case insensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
2nd Capturing Group
([a-z0-9-]{1,61})
Match a single character present in the list below
[a-z0-9-]
{1,61} matches the previous token between 1 and 61 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)
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)
\. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
3rd Capturing Group
([a-z0-9]{2,7})
Match a single character present in the list below
[a-z0-9]
{2,7} matches the previous token between 2 and 7 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)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

`
`
gm

Test String