Returns true for ECMA-262 whitespace characters (\s in ECMA regex): the union of ASCII
whitespace, \u{00a0}, and the Unicode space-separator category recognized by the spec.
Whether pattern is a syntactically valid ECMA-262 regular expression under the u flag, or
differs from one only by escaping non-alphanumeric characters.
Try to extract a simple prefix from a pattern like ^prefix.
Only matches patterns with alphanumeric characters, hyphens, underscores, and forward slashes.
The escaped form \/ is also accepted and normalised to /.