Regular Expressions (Regex) in JavaScript

Regular Expressions (Regex) help us search, match, and manipulate text patterns inside strings.

1. Basic Format of Regex

/pattern/flags

Example:

/hello/g

• / → Start and end of regex

• hello → pattern

• g → flag

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2. Flags

Flag	Meaning
g	Global → match all occurrences
i	Case insensitive
m	Multi-line

g → Global (match all occurrences)

const str = "cat cat dog";
console.log(str.match(/cat/g));

Output:

["cat", "cat"]

Without g, it would return only the first match.

i → Case insensitive

const str = "Cat cat CAT";
console.log(str.match(/cat/gi));

Output:

["Cat", "cat", "CAT"]

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3. + → One or More

/e+/g → Match at least one e

const str = "heeeello";
console.log(str.match(/e+/g));

Output:

["eeee"]

Explanation:

• + means one or more times.

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4. ? → Optional (0 or 1 time)

/e+a?/g

const str = "eeea eee";
console.log(str.match(/e+a?/g));

Output:

["eeea", "eee"]

Explanation:

• e+ → one or more e

• a? → a is optional

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5. * → Zero or More

/ea*/

const str = "e ea eaaa";
console.log(str.match(/ea*/g));

Output:

["e", "ea", "eaaa"]

Explanation:

• a* → zero or more a

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6. . → Any Single Character

/.at/

const str = "cat fat hat at";
console.log(str.match(/.at/g));

Output:

["cat", "fat", "hat"]

Explanation:

• . matches any one character before at.

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7. Escape Character \

/.\./

const str = "a.b c.d";
console.log(str.match(/.\./g));

Output:

["a.", "c."]

Explanation:

• \. means match actual dot (.).

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8. \w → Word Character

Matches:

• a-z

• A-Z

• 0-9

• underscore _

const str = "Hello_123!";
console.log(str.match(/\w/g));

Output:

["H","e","l","l","o","_","1","2","3"]

9. \s → Whitespace

const str = "Hello World";
console.log(str.match(/\s/g));

Output:

[" "]

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10. {min,max} → Quantifiers

/\w{1,3}/g

const str = "JavaScript";
console.log(str.match(/\w{1,3}/g));

Output:

["Jav", "aSc", "rip", "t"]

Explanation:

• Match minimum 1 and maximum 3 characters.

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11. Character Sets []

/[fc]at/

const str = "fat cat bat";
console.log(str.match(/[fc]at/g));

Output:

["fat", "cat"]

Explanation:

• Match either f or c before at.

12. [a-z] → Character Range

Explanation: Match any character between a and z.

const str = "ABCdef";
console.log(str.match(/[a-z]/g));

Output:

["d", "e", "f"]

13. (a|b) → OR Operator

Explanation: Match either a or b.

const str = "Tiger tiger";
console.log(str.match(/(T|t)/g));

Output:

["T", "t"]

Better way:

console.log(str.match(/t/gi));

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14. ^ → Start of Line

Explanation: Match only if string starts with pattern.

console.log(/^Hello/.test("Hello World"));

Output:

true