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Regular Expressions / Regular Expressions Advanced

Hey there! We're thoughtbot, a design and development consultancy that brings your digital product ideas to life. We also love to share what we learn.

This coding exercise comes from Upcase, the online learning platform we run. It's part of the Regular Expressions course and is just one small sample of all the great material available on Upcase, so be sure to visit and check out the rest.

Exercise Intro

Grouping is one of the most powerful aspects of regular expressions - it lets you match portions of the string and refer to them later, instead of having one big match.

In this problem, you'll be reformatting phone numbers into a standard format.

You can read more about grouping in the documentation or this blog post. You will also probably want to check the documentation for String#gsub.

Rubular is a fantastic website for getting quick feedback about your regex.


To start, you'll want to clone and run the setup script for the repo

git clone
cd regular-expressions-advanced

First, run bin/setup, then run rake to see the failing tests.

Make the tests pass one by one.

You can run rake at any point to re-run the tests.

Forum Discussion

If you find yourself stuck, be sure to check out the associated Upcase Forum discussion for this exercise to see what other folks have said.

Next Steps

When you've finished the exercise, head on back to the Regular Expressions course to find the next exercise, or explore any of the other great content on Upcase.


regular-expressions-advanced is Copyright © 2015-2018 thoughtbot, inc. It is free software, and may be redistributed under the terms specified in the LICENSE file.



This exercise is maintained and funded by thoughtbot, inc.

The names and logos for Upcase and thoughtbot are registered trademarks of thoughtbot, inc.

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