Regular expressions

The RegExp object is only as useful as your knowledge of Regular Expressions is strong. See here for an introductory primer, or see MDN for a more in-depth explanation.

Creating a RegExp Object

Standard Creation

It is recommended to use this form only when creating regex from dynamic variables.

Use when the expression may change or the expression is user generated.

var re = new RegExp(".*");

With flags:

var re = new RegExp(".*", "gmi");

With a backslash: (this must be escaped because the regex is specified with a string)

var re = new RegExp("\\w*");

Static initialization

Use when you know the regular expression will not change, and you know what the expression is before runtime.

var re = /.*/;

With flags:

var re = /.*/gmi;

With a backslash: (this should not be escaped because the regex is specified in a literal)

var re = /\w*/;

RegExp Flags

There are several flags you can specify to alter the RegEx behaviour. Flags may be appended to the end of a regex literal, such as specifying gi in /test/gi, or they may be specified as the second argument to the RegExp constructor, as in new RegExp('test', 'gi').

g - Global. Finds all matches instead of stopping after the first.

i - Ignore case. /[a-z]/i is equivalent to /[a-zA-Z]/.

m - Multiline. ^ and $ match the beginning and end of each line respectively treating \n and \r as delimiters instead of simply the beginning and end of the entire string.

6

u - Unicode. If this flag is not supported you must match specific Unicode characters with \uXXXX where XXXX is the character's value in hexadecimal.

y - Finds all consecutive/adjacent matches.

Matching With .exec()

Match Using .exec()

RegExp.prototype.exec(string) returns an array of captures, or null if there was no match.

var re = /([0-9]+)[a-z]+/;
var match = re.exec("foo123bar");

match.index is 3, the (zero-based) location of the match.

match[0] is the full match string.

match[1] is the text corresponding to the first captured group. match[n] would be the value of the nth captured group.

Loop Through Matches Using .exec()

var re = /a/g;
var result;
while ((result = re.exec('barbatbaz')) !== null) {
    console.log("found '" + result[0] + "', next exec starts at index '" + re.lastIndex + "'");
}

Expected output

found 'a', next exec starts at index '2'
found 'a', next exec starts at index '5'
found 'a', next exec starts at index '8'

Check if string contains pattern using .test()

var re = /[a-z]+/;
if (re.test("foo")) {
    console.log("Match exists.");
}

The test method performs a search to see if a regular expression matches a string. The regular expression [a-z]+ will search for one or more lowercase letters. Since the pattern matches the string, “match exists” will be logged to the console.

Using RegExp With Strings

The String object has the following methods that accept regular expressions as arguments.

  • "string".match(...
  • "string".replace(...
  • "string".split(...
  • "string".search(...

Match with RegExp

console.log("string".match(/[i-n]+/));
console.log("string".match(/(r)[i-n]+/));

Expected output

Array ["in"]
Array ["rin", "r"]

Replace with RegExp

console.log("string".replace(/[i-n]+/, "foo"));

Expected output

strfoog

Split with RegExp

console.log("stringstring".split(/[i-n]+/));

Expected output

Array ["str", "gstr", "g"]

Search with RegExp

.search() returns the index at which a match is found or -1.

console.log("string".search(/[i-n]+/));
console.log("string".search(/[o-q]+/));

Expected output

3
-1

Replacing string match with a callback function

String#replace can have a function as its second argument so you can provide a replacement based on some logic.

"Some string Some".replace(/Some/g, (match, startIndex, wholeString) => {
  if(startIndex == 0){
    return 'Start';
  } else {
    return 'End';
  }
});
// will return Start string End

One line template library

let data = {name: 'John', surname: 'Doe'}
"My name is {surname}, {name} {surname}".replace(/(?:{(.+?)})/g, x => data[x.slice(1,-1)]);

// "My name is Doe, John Doe"

RegExp Groups

JavaScript supports several types of group in it's Regular Expressions, capture groups, non-capture groups and look-aheads. Currently, there is no look-behind support.

Capture

Sometimes the desired match relies on it's context. This means a simple RegExp will over-find the piece of the String that is of interest, so the solution is to write a capture group (pattern). The captured data can then be referenced as...

  • String replacement "$n" where n is the n th capture group (starting from 1)
  • The n th argument in a callback function
  • If the RegExp is not flagged g, the n+1 th item in a returned str.match Array
  • If the RegExp is flagged g, str.match discards captures, use re.exec instead

Say there is a String where all + signs need to be replaced with a space, but only if they follow a letter character. This means a simple match would include that letter character and it would also be removed. Capturing it is the solution as it means the matched letter can be preserved.

let str = "aa+b+cc+1+2",
    re = /([a-z])\+/g;

// String replacement
str.replace(re, '$1 '); // "aa b cc 1+2"
// Function replacement
str.replace(re, (m, $1) => $1 + ' '); // "aa b cc 1+2"

Non-Capture

Using the form (?:pattern), these work in a similar way to capture groups, except they do not store the contents of the group after the match.

They can be particularly useful if other data is being captured which you don't want to move the indices of, but need to do some advanced pattern matching such as an OR

let str = "aa+b+cc+1+2",
    re = /(?:\b|c)([a-z])\+/g;

str.replace(re, '$1 '); // "aa+b c 1+2"

Look-Ahead

If the desired match relies on something which follows it, rather than matching that and capturing it, it is possible to use a look-ahead to test for it but not include it in the match. A positive look-ahead has the form (?=pattern), a negative look-ahead (where the expression match only happens if the look-ahead pattern did not match) has the form (?!pattern)

let str = "aa+b+cc+1+2",
    re = /\+(?=[a-z])/g;

str.replace(re, ' '); // "aa b cc+1+2"

Using Regex.exec() with parentheses regex to extract matches of a string

Sometimes you doesn't want to simply replace or remove the string. Sometimes you want to extract and process matches. Here an example of how you manipulate matches.

What is a match ? When a compatible substring is found for the entire regex in the string, the exec command produce a match. A match is an array compose by firstly the whole substring that matched and all the parenthesis in the match.

Imagine a html string :

<html>
<head></head>
<body>
  <h1>Example</h1>
  <p>Look a this great link : <a href="https://stackoverflow.com">Stackoverflow</a> http://anotherlinkoutsideatag</p>
  Copyright <a href="https://stackoverflow.com">Stackoverflow</a>
</body>

You want to extract and get all the links inside an a tag. At first, here the regex you write :

var re = /<a[^>]*href="https?:\/\/.*"[^>]*>[^<]*<\/a>/g;

But now, imagine you want the href and the anchor of each link. And you want it together. You can simply add a new regex in for each match OR you can use parentheses :

var re = /<a[^>]*href="(https?:\/\/.*)"[^>]*>([^<]*)<\/a>/g; 
var str = '<html>\n    <head></head>\n    <body>\n        <h1>Example</h1>\n        <p>Look a this great link : <a href="https://stackoverflow.com">Stackoverflow</a> http://anotherlinkoutsideatag</p>\n\n        Copyright <a href="https://stackoverflow.com">Stackoverflow</a>\n    </body>\';\n';
var m;
var links = [];

while ((m = re.exec(str)) !== null) {
    if (m.index === re.lastIndex) {
        re.lastIndex++;
    }
    console.log(m[0]); // The all substring
    console.log(m[1]); // The href subpart
    console.log(m[2]); // The anchor subpart

    links.push({
      match : m[0],   // the entire match
      href : m[1],    // the first parenthesis => (https?:\/\/.*)
      anchor : m[2],  // the second one => ([^<]*)
    });
}

At the end of the loop, you have an array of link with anchor and href and you can use it to write markdown for example :

links.forEach(function(link) {
  console.log('[%s](%s)', link.anchor, link.href);
});

To go further :

  • Nested parenthesis