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Explain 'dotAll' flag for regular expressions in JavaScript
The dotAll flag returns true or false depending upon if the s flag has been set in the regular expression. The s flag enables "dotAll" mode, where the dot . metacharacter matches any character, including newlines.
What is the dotAll Flag?
By default, the dot . in regular expressions matches any character except newlines. When the s flag is used, the dot can also match newline characters (\r, etc.), making it truly match "any" character.
Syntax
regex.dotAll // Returns true if 's' flag is set, false otherwise
Example: Checking dotAll Flag
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>dotAll Flag Example</title>
</head>
<body>
<h1>dotAll Flag for Regular Expressions</h1>
<div id="result" style="color: green; font-size: 18px; margin: 10px 0;"></div>
<button onclick="checkDotAll()">Check dotAll Flag</button>
<script>
function checkDotAll() {
const regexWithS = new RegExp("Hello", "s");
const regexWithoutS = new RegExp("Hello");
const result = document.getElementById("result");
result.innerHTML = `
<p>Regex with 's' flag - dotAll: ${regexWithS.dotAll}</p>
<p>Regex without 's' flag - dotAll: ${regexWithoutS.dotAll}</p>
`;
}
</script>
</body>
</html>
Example: Practical Usage with Newlines
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>dotAll Practical Example</title>
</head>
<body>
<h1>dotAll Flag - Matching Newlines</h1>
<div id="output" style="font-family: monospace; white-space: pre-line; margin: 10px 0;"></div>
<button onclick="testNewlineMatching()">Test Newline Matching</button>
<script>
function testNewlineMatching() {
const text = "Hello\nWorld";
const regexNormal = /Hello.World/;
const regexDotAll = /Hello.World/s;
const output = document.getElementById("output");
output.innerHTML = `
Text: "Hello\nWorld"
Normal regex /Hello.World/:
- Matches: ${regexNormal.test(text)}
- dotAll flag: ${regexNormal.dotAll}
DotAll regex /Hello.World/s:
- Matches: ${regexDotAll.test(text)}
- dotAll flag: ${regexDotAll.dotAll}
`;
}
</script>
</body>
</html>
Comparison
| Flag | dotAll Property | Dot Matches Newlines | Example |
|---|---|---|---|
| Without 's' | false |
No | /Hello.World/ |
| With 's' | true |
Yes | /Hello.World/s |
Key Points
- The
dotAllproperty is read-only and returns a boolean - When
true, the dot.matches any character including newlines - Useful for multiline text processing and pattern matching
- Can also be set using
new RegExp(pattern, "s")constructor
Conclusion
The dotAll flag is essential for detecting whether a regex can match newlines with the dot metacharacter. Use it to handle multiline text patterns effectively.
Updated on: 2026-03-15T23:18:59+05:30
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