CS133JS Beginning Programming: JavaScript

Regular Expressions

Table of Contents

String Methods that use Regular Expressions

match

These methods match a string against a string or a regular expression and return an array containing the matches, or null if no match is found.

• match using a string:

  let text = "Peter Piper picked a peck of pickled peppers.";
  let resultArray = text.match("pi");
  // resultArray: ["pi"], resultArray.index: 12
  

• match using regex. Note that this is more flexible, for example, we can ignore case and match globally.

  text = "Peter Piper picked a peck of pickled peppers.";
  resultArray = text.match(/pi/ig);  // ignore case
  // resultArray: ["Pi", "pi", pi"]
  

replace

This method searches a string using another string value or a regular expression. It returns a new string with the value(s) replaced. It does not change the original string.

• Replace a sub-string with another string:

  text = "Peter Piper picked a peck of pickled peppers.";
  result = text.replace("peck", "pint");
  // result: "Peter Piper picked a pint of pickled peppers."
  

• Replace using regex:

  text = "Once a duck, always a duck.";
  result = text.replace(/duck/g, "titan");
  // result: "Once a titan, always a titan."
  

search

This method matches a string against a regular expression. It returns the index (position) of the first match. If no match is found, it returns -1. The method is case sensitive.

let text = "Spring is springing";
let position = text.search(/spring/ig);
// position: 0

split

This method splits a string into an array of substrings. It returns a new array. It does not change the original string.

Note: if (" ") is used as separator, the string is split between words.

let text = "Are you a robot?";
const outputArray = text.split(/\s/);
// outputArray: ["Are", "you", "a", "robot?"]

Using Variables in a RegExp

Regular expression literals are constant, and can not be used with variables.

You can use variables with a RegExp constructor. For example:

let patternString = "( is){2}";
regExpObj = new RegExp(patternString, "g");
let result = regExpObj.test("It depends on what the meaning of is is");
// result: true

To insert a variable into a literal regex string (not RegExp object), just concatenate the variable into the appropriate part of the regex string. This will check to see if a string starts with the word in the variable.

let word = "cat";
regExpObj = new RegExp("^" + word, "i");
let result = regExpObj.test("Cats are mammals.");
// result: true

Reference

W3Schools

String match method

String replace method

String split method

Beginning JavaScript Lecture Notes by Brian Bird, 2022 are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.