?repeats the group contents below at most once:
\s*a Unicode whitespace character, repeated any number of times
extthe literal text ext (case sensitive) xthe literal x (12010 / 1708 / 7816) (case sensitive)
extthe literal text ext (case sensitive) .any Unicode character (except for line terminators)
\s*a Unicode whitespace character, repeated any number of times
\d{2,5}a Unicode decimal digit, repeated between 2 and 5 times