Regular Expressions, or Regex, are helpful tools used across many programming languages to find a specified combination of characters. Regex allows the programmer to search for any variation of a string matching a pattern instead of being limited to a specific, or "literal", query. For example, in a large database with multiple email address entries, a Regex pattern search of [any name]@[any domain].[any extension] will easily find all email addresses.

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• Usage
• Summary
• Credits

GIVEN a regex tutorial

• WHEN I open the tutorial
  • THEN I see a descriptive title and introductory paragraph explaining the purpose of the tutorial, a summary describing the regex featured in the tutorial, a table of contents linking to different sections that break down each component of the regex and explain what it does, and a section about the author with a link to the author’s GitHub profile
• WHEN I click on the links in the table of contents
  • THEN I am taken to the corresponding sections of the tutorial
• WHEN I read through each section of the tutorial
  • THEN I find a detailed explanation of what a specific component of the regex does
• WHEN I reach the end of the tutorial
  • THEN I find a section about the author and a link to the author’s GitHub profile

This tutorial explains the different components in the following regex:

Matching an Email

/^([a-z0-9_\.-]+)@([\da-z\.-]+)\.([a-z\.]{2,6})$/

Two ways to define a regular expression

let regexp = new RegExp("pattern", "flags");

let regexp = /pattern/; // no flags
let regexp = /pattern/gmi; // with flags g,m and i

• Slashes /.../ tell JavaScript that we are creating a regular expression. They play the same role as quotes for strings
• ^ is an anchor that matches from the start of input -a-z matches lowercase alpha characters
• 0-9 matches the numeric characters
• _ matches the literal '_' character
• . matches the literal '.' character
• - matches the literal '-' character
• @ matches the special character @
• \. matches the literal .
• $ is an anchor that matches to the end of the string

Note

A dot . on its own represents any character. In order to search for a dot within a string we need to 'escape' search by preponing the search for a dot with the backslash character. By using the \ it will make the following character a literal instead of a special character

Explanation for [a-z0-9_\.-] 

- matches all lowercase characters from a to z
- matches all numeric characters from 0 to 9
- matches special characters such as `_`, `-`, and `.`

Explanation for [\da-z\.-]

- `\d` matches for any decimal digit
- matches all lowercase characters from a to z
- matches special characters such as `.` and `-`

Explanation for [a-z\.]

- matches all lowercase characters from a to z
- matches special character, `.`

• Created by Amir Hackett