<a the literal text <a (case insensitive) Non-Capturing Group(?:(?!<a[^>]*>).)*? *?repeats the group contents below any number of times:
Negative Lookahead(?!<a[^>]*>) <athe literal text <a (case insensitive) Negated Character Class[^>]* *matches any number of characters outside the set below:
>the literal > (6210 / 768 / 3E16)
>the literal > (6210 / 768 / 3E16)
.any character (except for line terminators)
\$the literal $ (3610 / 448 / 2416)
\{the literal { (12310 / 1738 / 7B16)
Negated Character Class[^}]+ +matches at least one character outside the set below:
}the literal } (12510 / 1758 / 7D16)
\}the literal } (12510 / 1758 / 7D16)
.*?any character (except for line terminators), repeated any number of times
<the literal < (6010 / 748 / 3C16)
\/the literal / (4710 / 578 / 2F16)
a>the literal text a> (case insensitive) i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters