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Master all JavaScript regex flags: g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), y (sticky). With visual examples, common mistakes, and interview Q&A.
Flags modify how a regex pattern behaves. They come after the closing / delimiter in a literal regex (/pattern/flags) or as the second argument to RegExp(). You can combine multiple flags.
📊 All Regex Flags at a Glance
Example
FLAG │ NAME │ BEHAVIOR
──────┼──────────────────┼──────────────────────────────────────────────
g │ global │ Find ALL matches (not just the first)
i │ case-insensitive│ a-z matches A-Z (and vice versa)
m │ multiline │ ^ and $ match start/end of EACH LINE
s │ dotAll │ . matches newline characters too
u │ unicode │ Full Unicode support (required for \p{})
y │ sticky │ Match only from lastIndex position
│ │
gi │ global+insensitive │ All case-insensitive matches
gm │ global+multiline │ All matches across multiple lines
💻 Example 1 — g Flag (Global)
Example
const text = "JavaScript is great. JS runs everywhere. javascript is fun.";
// Without g — only finds the FIRST match
const first = text.match(/javascript/i);
console.log("First:", first[0]);
// With g — finds ALL matches
const all = text.match(/javascript/gi);
console.log("All:", all);
// replace with g — replaces all occurrences
console.log(text.replace(/javascript/gi, "JS"));
// replace without g — replaces only first
console.log(text.replace(/javascript/i, "JS"));
Output
First: JavaScript All: [ 'JavaScript', 'javascript' ] JS is great. JS runs everywhere. JS is fun. JS is great. JS runs everywhere. javascript is fun.
💻 Example 2 — i Flag (Case-Insensitive)
Example
// Without i — case-sensitive
console.log(/hello/.test("Hello")); // false
console.log(/hello/.test("HELLO")); // false
console.log(/hello/.test("hello")); // true
// With i — case-insensitive
console.log(/hello/i.test("Hello")); // true
console.log(/hello/i.test("HELLO")); // true
console.log(/hello/i.test("HeLLo")); // true
// Practical: case-insensitive search
const tags = ["JavaScript", "PYTHON", "java", "TypeScript", "JAVA"];
const javaRelated = tags.filter(tag => /java/i.test(tag));
console.log(javaRelated);
Output
false false true true true true [ 'JavaScript', 'java', 'TypeScript', 'JAVA' ]
💻 Example 3 — m Flag (Multiline)
js example
const multiline = `Line 1: First line
Line 2: Second line
Line 3: Third line`;
// Without m — ^ matches only start of WHOLE string, $ matches end of WHOLE string
const withoutM = multiline.match(/^Line \d:/g);
console.log("Without m:", withoutM);
// With m — ^ matches start of EACH line, $ matches end of EACH line
const withM = multiline.match(/^Line \d:/gm);
console.log("With m:", withM);
// Extract only lines ending with "line"
const endsLine = multiline.match(/.+line$/gim);
console.log("Lines ending with 'line':", endsLine);Output
Without m: [ 'Line 1:' ] With m: [ 'Line 1:', 'Line 2:', 'Line 3:' ] Lines ending with 'line': [ 'Line 1: First line', 'Line 2: Second line', 'Line 3: Third line' ]
💻 Example 4 — s Flag (dotAll — dot matches newlines)
Example
const html = `<div>
Hello
World
</div>`;
// Without s — . does NOT match newlines
const withoutS = html.match(/<div>.*<\/div>/);
console.log("Without s:", withoutS);
// With s — . DOES match newlines
const withS = html.match(/<div>.*<\/div>/s);
console.log("With s:", withS ? withS[0].slice(0, 30) + "..." : null);
// Another example
const sentence = "Hello\nWorld";
console.log(/Hello.World/.test(sentence)); // false — . won't match \n
console.log(/Hello.World/s.test(sentence)); // true — s flag allows \n
Output
Without s: null With s: <div> Hello World </div>... false true
💻 Example 5 — u Flag (Unicode) and y Flag (Sticky)
Example
// u flag — Unicode support
const emoji = "Hello 🌍 World 🚀";
// Without u — emoji counted wrong
console.log(emoji.match(/[\uD800-\uDBFF]/g)?.length); // may count surrogates
// With u — correct Unicode handling
console.log(emoji.match(/\p{Emoji}/gu)); // Unicode property escape
// y flag — sticky (must match at lastIndex)
const sticky = /\d+/y;
const str = "123abc456";
sticky.lastIndex = 0;
console.log(sticky.exec(str)); // matches "123" starting at 0
console.log(sticky.lastIndex); // 3 — moved past match
sticky.lastIndex = 3;
console.log(sticky.exec(str)); // null — position 3 is 'a', not digit
sticky.lastIndex = 6;
console.log(sticky.exec(str)); // matches "456" starting at 6
Output
null [ '🌍', '🚀' ] [ '1', '2', '3', index: 0, input: '123abc456', groups: undefined ] 3 null [ '4', '5', '6', index: 6, input: '123abc456', groups: undefined ]
📋 Flag Combinations in Real Use
Example
// gi — case-insensitive global search
"Cat cat CAT".match(/cat/gi); // [ 'Cat', 'cat', 'CAT' ]
// gm — find all line-starting words
`hello\nworld`.match(/^\w+/gm); // [ 'hello', 'world' ]
// gis — global, case-insensitive, dot matches newline
`<p>Hello\nWorld</p>`.match(/<p>.*?<\/p>/gis);
// gi with replace — replace all case-insensitive
"JS js Js".replace(/js/gi, "JavaScript"); // "JavaScript JavaScript JavaScript"
⚠️ Common Mistakes
❌ Mistake 1 — Using g Flag with test() in a Loop (stateful lastIndex)
Example
const re = /\d+/g;
const texts = ["abc 123", "def 456", "ghi 789"];
// BAD — RegExp with g is stateful! lastIndex persists between calls
texts.forEach(t => console.log(re.test(t)));
Output
true false true
Thegflag makes regex stateful —lastIndexadvances after eachtest(). Alternate calls give inconsistent results. Fix: create a new regex inside the loop, or use/pattern/.test(str)(fresh literal each time).
❌ Mistake 2 — Forgetting m Flag for Multiline Line Start/End
Example
const log = "ERROR: disk full\nWARN: low memory\nINFO: started";
console.log(log.match(/^ERROR.*/)); // null without m!
console.log(log.match(/^ERROR.*/m)); // [ 'ERROR: disk full' ]
Output
null [ 'ERROR: disk full' ]
❌ Mistake 3 — Using s Flag Without Checking Browser Support
Thes(dotAll) flag was introduced in ES2018. For older environments (Node < 10, older browsers), use[\s\S]as a workaround for matching newlines:/[\s\S]*/instead of/.*/s.
🎯 Key Takeaways
g— find all matches, not just the first; also makes regex stateful (updateslastIndex)i— case-insensitive matchingm—^and$match start/end of each line instead of the whole strings— dotAll:.matches newlines too (ES2018+)u— full Unicode support; required for\p{...}Unicode property escapesy— sticky: matches only fromlastIndexposition- Flags can be combined:
gi,gm,gis, etc.
❓ Interview Questions
Q1. What does the g flag do in JavaScript regex? > The g (global) flag makes the pattern find all matches in a string instead of stopping at the first. Methods like match() with g return an array of all matches. It also makes the regex stateful — lastIndex advances after each match with exec() or test().
Q2. What is the difference between /pattern/i and /pattern/? > /pattern/i is case-insensitive — a matches both a and A. Without i, matching is case-sensitive by default.
Q3. When do you need the m flag? > When working with multiline strings and you want ^ and $ to match the start and end of each individual line, not just the beginning and end of the entire string.
Q4. What problem can the g flag cause with test() in a loop? > A regex with g is stateful — it updates lastIndex after each test(). In a loop, consecutive test() calls alternate between true and false as lastIndex advances. Fix: use a fresh regex literal inside the loop, or avoid g with test().
Q5. What does the s (dotAll) flag do? > The s flag makes the dot . match any character including newline (\n, \r). By default, . does NOT match newlines.
Q6. What is the u flag used for? > The u flag enables full Unicode mode, allowing correct handling of characters outside the Basic Multilingual Plane (emoji, some Asian scripts). It is required for Unicode property escapes (\p{Letter}, \p{Emoji}).
Q7. Can you use multiple flags together? > Yes — combine them freely after the closing delimiter: /pattern/gi (global + case-insensitive), /pattern/gm (global + multiline), /pattern/gis (global + case-insensitive + dotAll).
Q8. What is the y (sticky) flag? > The sticky flag forces the regex to match starting exactly at lastIndex — it won't skip ahead to find a match. Useful for tokenizers that process a string sequentially without backtracking.
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for JavaScript Regex Flags – g, i, m, s, u, y Flags Explained with Examples.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
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