Как сопоставить строку, начинающуюся с нескольких символов (может быть вложенной) через регулярное выражение?

Случай 2
Захват только домена после ://

Regex:
(?<=:\/\/)[^\n|\/|:]+

Объяснение:
Positive Lookbehind (?<=:\/\/)
Assert that the Regex below matches
[1142] : matches the character : literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
Match a single character not present in the list below [^\n|\/|:]+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
\n matches a line-feed (newline) character (ASCII 10) [1121]
| matches the character | literally (case sensitive)
] \/ matches the character / literally (case sensitive)
|: matches a single character in the list |: (case sensitive)

Случай 3:
Захват домена после ://, если нет @ в тексте, и если @ присутствует в тексте, запишите текст после этого.

Регулярное выражение:
(?!:\/\/)(?:[A-z]+\.)*[A-z][A-z]+\.[A-z]{2,}

Объяснение:

    Negative Lookahead (?!:\/\/)
Assert that the Regex below does not match
: matches the character : literally (case sensitive)
\/ matches the character / literally (case sensitive)
\/ matches the character / literally (case sensitive)
Non-capturing group (?:[A-z]+\.)*
* Quantifier — Matches between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below [A-z]+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
A-z a single character in the range between A (index 65) and z (index 122) (case sensitive)
\. matches the character . literally (case sensitive)
Match a single character present in the list below [A-z]
A-z a single character in the range between A (index 65) and z (index 122) (case sensitive)
Match a single character present in the list below [A-z]+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
A-z a single character in the range between A (index 65) and z (index 122) (case sensitive)
\. matches the character . literally (case sensitive)
Match a single character present in the list below [A-z]{2,}
{2,} Quantifier — Matches between 2 and unlimited times, as many times as possible, giving back as needed (greedy)
A-z a single character in the range between A (index 65) and z (index 122) (case sensitive)