JavaScript RegExp dotAll Property

The dotAll property in JavaScript regular expressions determines whether the 's' (dotAll) flag is enabled. This flag modifies the behaviour of the dot (.) metacharacter, allowing it to match any character, including line terminators like \n, \r, and others. Without the s flag, the dot (.) matches any character except line breaks.

The dotAll property is a read-only Boolean that reflects whether the s flag is active in a given regular expression.

// Regular expression without 's' flag
let regex1 = /a.b/;
console.log(regex1.dotAll);
console.log(regex1.test("a\nb")); 

// Regular expression with 's' flag
let regex2 = /a.b/s;
console.log(regex2.dotAll);
console.log(regex2.test("a\nb"));

false
false
true
true

• Without s flag: The dot (.) does not match line breaks, so "a\nb" fails to match.
• With s flag: The dot (.) matches line breaks, allowing "a\nb" to match successfully.

Syntax

regex.dotAll

Key Points

• Read-Only: Indicates whether the s flag is present in the regex.
• Enhanced Matching: When the s flag is set, the dot (.) matches all characters, including line terminators.
• Default Behavior: By default, dotAll is false unless explicitly enabled with the s flag.

Real-World Examples of the dotAll Property

1. Matching Multiline Strings

let s = "Hello\nWorld!";
let regex = /Hello.World/s;

console.log(regex.test(s));

true

With the s flag, the dot (.) matches the newline character, allowing the regex to match the entire "Hello\nWorld!" string.

2. Extracting Text Across Lines

let s = `Start
Middle
End`;

let regex = /Start.*End/s;

console.log(regex.test(s));
console.log(s.match(regex)[0]);

true
Start
Middle
End

The .* pattern, combined with the s flag, matches everything from "Start" to "End", including newlines.

3. Simple Multiline Match

let s = "abc\ndef";
let regex = /a.c/s;

console.log(regex.test(s));

true

4. Finding Content Between Tags

let html = "<div>\n  Content\n</div>";
let regex = /<div>.*<\/div>/s;

console.log(html.match(regex)[0]);

<div>
  Content
</div>

5. Matching Entire Multiline Blocks

let log = `Error: Something went wrong.
Details:
    Line: 42
    File: app.js`;

let regex = /Error:.*?app\.js/s;

console.log(regex.test(log));

true

6. Comparing dotAll and Multiline (m) Flags

• s (dotAll): Allows . to match newlines.
• m (multiline): Modifies ^ and $ to match the start and end of each line.

let s = "line1\nline2";

// Using 's'
let sRegex = /.+/s;
console.log(sRegex.test(s));

// Using 'm'
let mRegex = /^line2/m;
console.log(mRegex.test(s));

true
true

Why Use the dotAll Property?

• Simplified Matching: Avoid the need for explicit patterns like [\s\S] to match line terminators.
• Cleaner Regex: Makes regular expressions more readable and concise for multiline data.
• Versatile Use: Ideal for processing logs, HTML, or any content with potential line breaks.

Recommended Links:

• JavaScript RegExp Complete Reference
• Javascript Cheat Sheet-A Basic guide to JavaScript
• JavaScript Tutorial