SRL – Simple Regex Language
The first example:    /^(?:[0-9]|[a-z]|[\._%\+-])+(?:@)(?:[0-9]|[a-z]|[\.-])+(?:\.)[a-z]{2,}$/i

is a total strawman, needlessly obfuscated.  How about writing it like this:    /^[0-9a-z._%+-]+@[0-9a-z.-]+.[a-z][a-z]+$/i

which, while "scary looking", is at least immediately readable by anyone who knows even the basics about REs.  If the argument for "verbose REs" is valid, it ought to stand up at least a typical standard RE.Also, it's not clear that "letter" and "[a-z]" mean the same thing.  Does "letter" include uppercase?  Does it include non-ASCII letters like "[[:alpha:]]" does?  Don't forget the weird collation behavior "[a-z]" sometimes encounters.