Generate the simplest string that matches a given regular expression.

npm install simpexp
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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



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