std/regex/syntax
std/regex/src/syntax.trb
The syntax of a pattern: RE2's, read into a tree of RegexNodes. What RE2 does not have - backreferences and
lookaround - is refused with a message that says so, because a regular expression here runs in time linear in its
input.
type RegexError
type RegexError with Show, ErrorWhy a text is not a regular expression: what is wrong, and the byte offset in the pattern where it was found.