For more information on REXS' syntax (or how to use it), take a look at https://esolangs.org/wiki/REXS.
An example usage of REXS can be to match on any URL that starts with http:// or https://, then match any subdomains, followed by the domain and .com:
assert(START);
match("http");
repeat(0, 1) {
match("s");
}
match("://");
repeat(0, inf, nongreedy) {
repeat(1, inf, nongreedy) {
match(ANY);
}
match(".");
}
group() {
repeat(1, inf, nongreedy) {
match(ANY);
}
match(".com");
}
assert(END);
This example will be compiled to /^https?:\/\/(?:.+?\.)*?(.+?\.com)$/
.