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Build Your Own Regex

This is a test spec driven guide to help you build a simple regex engine.

This regex engine supports the following syntax:

Syntax Meaning Example matches
a Matches the specified character literal q q
* Matches 0 or more of the previous character a* "", a, aa, aaa
? Matches 0 or 1 of the previous character a? "", a
. Matches any character literal . a, b, c, d, e ...
^ Matches the start of a string ^c c, ca, caa, cbb ...
$ Matches the end of a string a$ ba, baaa, qwerta ...

The goal is to provide a syntax robust enough to match a large portion of regex use cases with minimal code. The included solution is under 40 LOC.

This repo has an accompanying blog post that explains the solution here


npm install
npm test

Now simply make changes to the regex.js file until the tests pass. Note that the tests are all pending the begin with. Just change the xdescribes to describes to switch the tests on.


This project requires a strong understanding of recursion and wonderfully showcases it's elegance with a non-trivial example.


I was inspired by Rob Pike's original implementation of this program in c and Peter Norvig's rendition of it in Udacity's "Design of Computer Programs" course.


This is a test spec driven guide to help you build a simple regex implementation



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