When not to use a regex August 13, 2017 on Drew DeVault's blog

The other day, I saw Learn regex the easy way. This is a great resource, but I felt the need to pen a post explaining that regexes are usually not the right approach.

Let’s do a little exercise. I googled “URL regex” and here’s the first Stack Overflow result:

https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{2,256}\.[a-z]{2,6}\b([-a-zA-Z0-9@:%_\+.~#?&//=]*)

source

This is a bad regex. Here are some valid URLs that this regex fails to match:

Here are some invalid URLs the regex is fine with:

This answer has been revised 9 times on Stack Overflow, and this is the best they could come up with. Go back and read the regex. Can you tell where each of these bugs are? How long did it take you? If you received a bug report in your application because one of these URLs was handled incorrectly, do you understand this regex well enough to fix it? If your application has a URL regex, go find it and see how it fares with these tests.

Complicated regexes are opaque, unmaintainable, and often wrong. The correct approach to validating a URL is as follows:

from urllib.parse import urlparse

def is_url_valid(url):
    try:
        urlparse(url)
        return True
    except:
        return False

A regex is useful for validating simple patterns and for finding patterns in text. For anything beyond that it’s almost certainly a terrible choice. Say you want to…

validate an email address: try to send an email to it!

validate password strength requirements: estimate the complexity with zxcvbn!

validate a date: use your standard library! datetime.datetime.strptime

validate a credit card number: run the Luhn algorithm on it!

validate a social security number: alright, use a regex. But don’t expect the number to be assigned to someone until you ask the Social Security Administration about it!

Get the picture?

Have a comment on one of my posts? Start a discussion in my public inbox by sending an email to ~sircmpwn/public-inbox@lists.sr.ht [mailing list etiquette]

Articles from blogs I read Generated by openring

Summary of changes for February

Hey everyone! This is the list of all the changes we've done to our projects and apps during the month of February. We'll also be reporting in our on position in the world, and on our future plans. Summary Of Changes Uxn, started working on a fa…

via Hundred Rabbits February 28, 2021

Contexts and structs

Introduction In many Go APIs, especially modern ones, the first argument to functions and methods is often context.Context. Context provides a means of transmitting deadlines, caller cancellations, and other request-scoped values across A…

via The Go Programming Language Blog February 24, 2021

Status update, February 2021

Hi! Once again my focus has been Wayland-related projects this month. A steady stream of improvements made it into wlroots: Xyene made the X11 clipboard code a lot more robust, bl4ckbone has made good progress on the upcoming Pixman-based software renderer, a…

via emersion February 22, 2021