JavaScript

15Regular Expression Updates

WHAT'S IN THIS CHAPTER?

• The flags property
• The y, u, and s flags
• Named capture groups
• Lookbehind assertions
• Unicode code point escapes
• Unicode property escapes

CODE DOWNLOADS FOR THIS CHAPTER

You can download the code for this chapter at https://thenewtoys.dev/bookcode or https://www.wiley.com/go/javascript-newtoys.

In this chapter, you'll learn about the many new regular expression features added in ES015 and ES2018, including a flags property on instances reflecting all of the flags for the instance; named capture groups for more readable, maintainable regular expressions; lookbehind assertions; and Unicode property escapes providing access to powerful matching classes.

THE FLAGS PROPERTY

In ES2015, regular expressions got a flags accessor property that returns a string containing the flags for the expression. Prior to the flags property, the only way to know what flags the RegExp object had was to look at the individual properties reflecting its individual flags (rex.global, rex.multiline, etc.), or use its toString method and look at the end of the string. The flags property makes them available directly as a string:

const rex = /example/ig;console.log(rex.flags); // "gi"

The specification defines that the flags are provided in alphabetical order, regardless of how they were specified when the expression was created: gimsuy. (You'll learn about those three new ones, s, u, and y, in the following sections.) You can see the ...