In JavaScript, a RegExp is a pattern that describes text. You write one between slashes: /cat/. That pattern matches the letters c, a, t in that order.
test() returns true or false. exec() returns the matched text and the index where it started, or null when nothing matches. A flag changes the match: i ignores letter case, and g finds every match. ^ and $ pin the pattern to the whole string.
This page teaches how a JavaScript RegExp works. The tables further down list every pattern guide in this tutorial, including test(), exec(), \d, and +.
/cat/.test("catch") is true. /^cat$/.test("catch") is false.
A literal and the constructor build the same kind of object:
const literal = /cat/i;
const built = new RegExp("cat", "i");
literal.test("Cat"); // true
built.exec("The Cat sat"); // match details
const digits = /\d+/;
const same = new RegExp("\\d+");Ideas every JavaScript RegExp tutorial builds on:
| Idea | Example | Remember |
|---|---|---|
| Literal | /cat/i | Pattern and flags between slashes |
| Constructor | new RegExp("cat", "i") | Use this when the pattern is a string |
| Yes or no | pattern.test(text) | Returns true or false |
| The match | pattern.exec(text) | Text and index, or null |
| Whole string | /^cat$/ | ^ is the start, $ is the end |
| A digit | /\d+/ | One or more digits |
Three short programs that show how a RegExp works in a JS tutorial.
^ and $ mean the pattern must cover every character.
const whole = /^cat$/;
console.log(whole.test("cat"));
console.log(whole.test("catch"));
console.log(/cat/.test("catch"));"cat" and nothing else, so the first call is true.false."catch", so it is true.The i flag matches Cat and cat. exec() returns the text it found.
const found = /cat/i.exec("The Cat sat");
console.log(found[0]);
console.log(found.index);Cat because i ignores letter case."Cat".4. The match starts after "The ".\d+ is one or more digits. The g flag lets replace change every hit.
console.log(/\d+/.test("room 12"));
console.log("ab12cd".match(/\d+/)[0]);
console.log("hello".replace(/l/g, "L"));12 inside "room 12", so test is true.0 is "12".l characters, so "hello" becomes "heLLo".Every RegExp guide in this JavaScript series (44 tutorials). Open one when you need that token.
Ask a pattern a yes-or-no question, or read the text it found.
| Pattern | Description | Tutorial |
|---|---|---|
test() | Return true or false when a pattern matches anywhere in a string. | Open |
exec() | Run a match and return capture groups, index, and input — or null. | Open |
toString() | Return the /pattern/flags string representation of a RegExp. | Open |
compile() | Legacy method to recompile an existing RegExp (prefer new RegExp). | Open |
Read flags on the RegExp object. These are values, not function calls.
| Pattern | Description | Tutorial |
|---|---|---|
constructor | Points to the RegExp function that created the regex object. | Open |
global | True when the g flag is set — find all matches, not just the first. | Open |
ignoreCase | True when the i flag is set — case-insensitive matching. | Open |
Match one character from a set, a range, or either of two patterns.
| Pattern | Description | Tutorial |
|---|---|---|
[0-9] | Character class matching any digit from 0 through 9. | Open |
[abc] | Match any one character from the set inside the brackets. | Open |
[^0-9] | Negated class — match any character that is not a digit. | Open |
[^abc] | Negated class — match any character not in the set. | Open |
(x|y) | Alternation — match either the left or right sub-pattern. | Open |
Short tokens for a digit, whitespace, a word character, or an escape.
| Pattern | Description | Tutorial |
|---|---|---|
\b | Word boundary between a word and non-word character. | Open |
\r | Carriage return character. | Open |
\d | Shorthand for any digit — equivalent to [0-9]. | Open |
. | Wildcard — matches any character except newline (by default). | Open |
\f | Form feed character. | Open |
\n | Line feed (newline) character. | Open |
\D | Any non-digit character. | Open |
\S | Any non-whitespace character. | Open |
\W | Any non-word character (not letter, digit, or _). | Open |
\B | Not a word boundary — opposite of \b. | Open |
\0 | Null character (U+0000). | Open |
\s | Whitespace — spaces, tabs, newlines, and similar. | Open |
\t | Horizontal tab character. | Open |
\uXXXX | Unicode code point by four hex digits. | Open |
\v | Vertical tab character. | Open |
\w | Word character — letters, digits, and underscore. | Open |
\xHH | Byte value by two hex digits. | Open |
\ooo | Byte value by one to three octal digits (legacy). | Open |
Change the match: every hit, any letter case, or each line.
| Pattern | Description | Tutorial |
|---|---|---|
d (indices) | HasIndices flag — exec results include match indices. | Open |
g (global) | Global flag — continue searching after each match. | Open |
i (ignoreCase) | Ignore case when matching letters. | Open |
m (multiline) | Multiline flag — ^ and $ match line boundaries. | Open |
Pin a match to the start or end, or peek ahead without taking those characters.
Say how many times the previous token may repeat.
| Pattern | Description | Tutorial |
|---|---|---|
+ | One or more repetitions of the preceding token. | Open |
? | Zero or one repetition — makes the preceding token optional. | Open |
* | Zero or more repetitions of the preceding token. | Open |
{X} | Exactly X repetitions of the preceding token. | Open |
{X,} | At least X repetitions (no upper limit). | Open |
{X,Y} | Between X and Y repetitions inclusive. | Open |
Open the yes-or-no guide—syntax, examples, and FAQs.
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