Dynamic Typing: Regular Strings

Felix supports subtyping of strings by regular expressions.

The typing can be enforced at run time, and uses a fast deterministic finite state automaton with complexity linear in the string length.

Note regular string types themselves are static types known to the compiler.

Regular expressions can be formed using the below combinators.
They can also be named using a regexp definition.

Regular Combinators
Name	Sample Type annotation
Kleene closure	letter *
Alternation	letter \| digit
Catenation	digit digit digit
Option	letter ?
One or More	letter +

Charsets
Name	Sample Type annotation
Any	_
Not newline	.
One of	["abc" "de" 30]
Any but	[^"abcde"]
Range	["a"-"z"]

Example Program

include "std";

regexp lower = ["abcdefghijklmnopqrstuvwxyz"];
regexp upper = ["A"-"Z"];
regexp digit = ["0123456789"];
regexp alpha = lower | upper | "_";
regexp id = alpha (alpha | digit) *;


print 
  regmatch "identifier" with
  | digit+ => "Number"  
  | id =>  "Identifier"  endmatch
;
endl;