Expand regex for absolute IRIs

[balhoff]

Fixes #176.

I updated to the regex to match the grammar in https://www.ietf.org/rfc/rfc2396.txt:

Scheme names consist of a sequence of characters beginning with a
lower case letter and followed by any combination of lower case
letters, digits, plus ("+"), period ("."), or hyphen ("-"). For
resiliency, programs interpreting URI should treat upper case letters
as equivalent to lower case in scheme names (e.g., allow "HTTP" as
well as "http").

 scheme        = alpha *( alpha | digit | "+" | "-" | "." )

[RubenVerborgh]

Thanks @balhoff, good catch! Your code is correct; I just slightly changed the order from a-z0-9.-+ to a-z0-9.+- so readers don't confuse it as "a to z, 0 to 9, dot to plus".

[balhoff]

Thanks @balhoff, good catch! Your code is correct; I just slightly changed the order from a-z0-9.-+ to a-z0-9.+- so readers don't confuse it as "a to z, 0 to 9, dot to plus".

Good idea, thank you!