Generate the simplest string that matches a given regular expression.


simpexp.js is a fork of randexp.js that will generate a simplest (shortest) string that matches a given RegExp, in a deterministic way (so that the same regex always generates the same string).


var SimpExp = require('simpexp');
// supports grouping and piping 
new SimpExp(/hello+ (world|to you)/).gen();
// => hello world 
// shorter is better 
new SimpExp(/hello+ (world|to u)/).gen();
// => hello to u 
// a real-life example 
new SimpExp(/<\s*br(?:[\s/]*|\s[^>]*)>/i).gen();
// => <br> 
// sets and ranges and references 
new SimpExp(/<([a-z]\w{0,20})>foo<\1>/).gen();
// => <a>foo<a> 
// dynamic regexp shortcut 
new SimpExp('(sun|mon|tue|wednes|thurs|fri|satur)day', 'i');
// is the same as 
new SimpExp(new RegExp('(sun|mon|tue|wednes|thurs|fri|satur)day', 'i'));

If you're only going to use gen() once with a regexp and want slightly shorter syntax for it

var simpexp = require('simpexp').simpexp;
simpexp(/[1-6]/); // 1 
simpexp('great|good( job)?|excellent'); // good 

If you miss the old syntax

/yes|no|maybe|i don't know/.gen(); // no 


Generating a simplest string that matches a given regex helps you capture the essence of the regex, and therefore understand what it does. Additionally, when a regex is created to match various variations of a pattern, the simplest string that matches the regex is often the most natural form of the pattern.

This tool can also help you check correctness of complex regular expressions. By trying to minimize the length of the match, it may help expose loopholes in your regex that allow unintended strings to be matched.


Special thanks to Roly Fentanes and his randexp.js library, on which this little library is based. You can also find more information there.


npm install simpexp