There's nothing wrong with regex syntax. But there _is_ something wrong with the formatting of your example: it's not readable. Perhaps you _should_ write (assu

There's nothing wrong with regex syntax. But there _is_ something wrong with the formatting of your example: it's not readable. Perhaps you _should_ write (assuming Java-syntax): (?x: #standard token is an uppercase letter, digit, dot, or hyphen \b [A-Z0-9._%-]+ #1 or more of standard token, underscore, or percent sign @ #at-sign [A-Z0-9.-]+ #1 or more standard tokens \. #dot [A-Z]{2,4} #2 to 4 uppercase letters \b ) We can easily optimize for readability with regex syntax.