I'm not an expert in regexes in other platforms, but already if you want a common subset between Java and JS, you're limited to the following constructs: * Lit

I'm not an expert in regexes in other platforms, but already if you want a common subset between Java and JS, you're limited to the following constructs: * Literal characters * Some escape sequences for literal characters: \t \r \n \xHH \uUUUU as well as the ones for the regex metacharacters such as \[ \( etc. * Groups, sequences and alternatives * Look-ahead (positive and negative) * Look-behind, if supported (ES 2018+) * Back-references to existing groups opened before the back-references, but you need to make sure that they're not syntactically followed by a digit, otherwise parsing is different between JS and Java * Greedy and lazy quantifiers * Custom character classes with [], without intersections or inner custom character classes That's assuming you have 'u' support in JS and that you use it (ECMAScript 2015), and that you don't use any other flag. (you can use the 'g' flag in JS or not; it affects the behavior of the methods, but not of the regex itself). If you don't have 'u' support, it's only the same if you restrict your regex and your input to ASCII characters. Some things that are unexpectedly NOT portable: * Most pre-defined character classes like \s and the like * CASE_INSENSITIVE/'i' and MULTILINE/'m' * \p{...} character classes * \b and \B (there's a bug in OpenJDK: https://bugs.openjdk.java.net/browse/JDK-8264160 https://bugs.openjdk.java.net/browse/JDK-8264160)