?repeats the group contents below at most once:
\s*a Unicode whitespace character, repeated any number of times
Xthe literal X (8810 / 1308 / 5816) (case sensitive)
xthe literal x (12010 / 1708 / 7816) (case sensitive)
/the literal / (4710 / 578 / 2F16)
.the literal . (4610 / 568 / 2E16)
*the literal (3210 / 408 / 2016) , repeated any number of times
\d+a Unicode decimal digit, repeated at least once
Non-Capturing Group(?:\.\d+)? ?repeats the group contents below at most once:
\.the literal . (4610 / 568 / 2E16)
\d+a Unicode decimal digit, repeated at least once
"?optionally matches the literal " (3410 / 428 / 2216)