The dotAll mode in regular expressions translates this sentence into English.

ECMAScript 9 (ES2018) introduced the dotAll mode for regular expressions, addressing the limitation where the . character could not match line terminators through the s flag. This feature simplifies multiline text processing, particularly when handling template strings or text blocks containing line breaks.

Limitations of the Dot (.) in Traditional Mode

In traditional regular expressions, the metacharacter . matches any single character except line terminators (\n, \r, \u2028, \u2029). For example:

const regex = /a.b/;
console.log(regex.test('aXb'));  // true
console.log(regex.test('a\nb')); // false

This design prevented . from directly matching content across lines, forcing developers to use workarounds like [\s\S]:

const regex = /a[\s\S]b/;
console.log(regex.test('a\nb')); // true

Syntax and Features of dotAll Mode

ES9 introduced the s flag to enable dotAll mode, allowing . to match all characters, including line terminators:

const regex = /a.b/s;
console.log(regex.test('aXb'));  // true
console.log(regex.test('a\nb')); // true

Detecting dotAll Mode in Regular Expressions

The flags property can be used to check if a regular expression has the s flag:

const regex = /./s;
console.log(regex.flags.includes('s')); // true

Practical Use Cases

Matching Multiline Template Strings

dotAll mode significantly simplifies code when processing template strings with line breaks:

const text = `
  Name: John
  Age: 30
`;
const regex = /Name:\s+(.*)\s+Age:\s+(.*)/s;
const match = text.match(regex);
console.log(match[1], match[2]); // "John", "30"

Extracting HTML Tag Content

Matching HTML tag content that may contain line breaks:

const html = '<div>Line 1\nLine 2</div>';
const regex = /<div>(.*?)<\/div>/s;
console.log(html.match(regex)[1]); // "Line 1\nLine 2"

Interaction with Other Flags

Difference from the m Flag

• m (multiline mode): Only alters the behavior of ^ and $
• s (dotAll mode): Alters the behavior of .

They can be combined:

const regex = /^a.+b$/ms;
console.log(regex.test('aX\nYb')); // true

Compatibility with the u Flag

The s flag is fully compatible with Unicode mode (u):

const regex = /./su;
console.log(regex.test('\u{1F600}')); // true

Performance Considerations

While dotAll mode offers convenience, it may impact performance in extreme cases:

// Example that may cause catastrophic backtracking
const regex = /<.*>/s;
console.log(regex.test('<a>\n<b>\n<c>'));

For complex scenarios, it's recommended to use more precise patterns:

// More efficient alternative
const regex = /<[^>]+>/;

Browser and Runtime Support

Modern JavaScript environments widely support the s flag:

// Feature detection
const isDotAllSupported = () => {
  try {
    new RegExp('.', 's');
    return true;
  } catch (e) {
    return false;
  }
};

Node.js has fully supported this feature since v10.0.0, and mainstream browsers implemented it in versions released after 2018.