from regstring import pyregstring

r = b"(?<action>\\w+)\\s(?<url>\\S+)\\s(?<version>\\S+)\r\n"

rs = pyregstring()

if (rs.parse_regex(r)):
    rs.set(b"action", b"GET")
    rs.set(b"url", b"/test")
    rs.set(b"version", b"HTTP/1.1")
    print(rs.to_bytes())
    print(rs.names())
    print(rs.size())

b'GET /test HTTP/1.1\r\n'
[b'action', b'url', b'version']
20