Text Processing with JavaScript

Recipe 50

Testing a Pattern with the Negative Lookbehind

Task

Suppose you’re searching hospital records for a patient named Smith, but most of the data you get from search results is about Dr. Smith. You need a regex that matches “Smith” but excludes “Dr. Smith.”

Solution

Use a negative lookbehind assertion denoted by (?<! ... ):

part_2/negative_lookbehind/negative_lookbehind_ex1.js

const re = /(?<!Dr\.\s)Smith/;

console.log(re.test("Dr. Smith"));     // → false
console.log(re.test("Mr. Smith"));     // → true
console.log(re.test("John Smith"));    // → true

Your regex now finds all records containing “Smith” except for “Dr. Smith.”

Discussion

The negative version of lookbehind asserts that the pattern within the lookbehind does ...