sQVe / talk-regexp

Let's talk about Regular Expressions

Regular Expressions

What is it?

A regular expression is a sequence of characters or tokens that define a search pattern.

The search pattern is usually used for search and replace operations or validation of strings.

// Example use of RegExp.
const tabsToSpaces = str => str.replace(/\t/g, '  ')

History

Created in 1951 and entered popular use from 1968 due to adoption by many Unix applications / tools like: QED, grep, vi, sed, AWK etc.

Flavors

There are many flavors of RegExp due to the heavy reimplementation of RegExp engines in different languages. Most of the favors are "Perl compliant" though, which is a syntax that dates as far back as 1987.

State of JavaScript RegExp

ECMAScript 2018 and future proposals have fixed plenty of JavaScript's well-known RegExp deficiencies:

• DOTALL flag.
• Look-behind assertion support.
• Named groups.
• Unicode property escapes.

API

Methods in JavaScript that utilize RegExp:

• RegExp
  • exec
  • test
• String
  • match
  • matchAll
  • search
  • replace
  • split

Common pitfalls

These are some common pitfalls when using RegExp with JavaScript:

• Exec is faster than match when using a RegExp without the global flag.
• No group matching when using the global flag.
• Split adds the matched groups to the result.
• Too narrow and greedy.
  • \w is [a-zA-Z0-9] not [all possible characters].
  • Greedy by default. Lazy mode/token is confusing.

Resources

Useful resources to learn RegExp:

• Cheatsheet
• RegEx101
• RegExCrossword
• RegExGolf