MediumNumbers3 min read

Hexadecimal Number Regex

Validates a base-16 (hexadecimal) number that must be written with an explicit '0x' or '0X' prefix, followed by one or more hex digits.

#hexadecimal#hex#numbers#base-16#validation#regex

Regex Pattern

^0[xX][0-9a-fA-F]+$

Pattern Breakdown

Hover over a token to see what it does.

^0[xX][0-9a-fA-F]+$

Token	Meaning
^	Anchors the match to the start of the string
0[xX]	Literal '0' followed by either lowercase 'x' or uppercase 'X', matching the required hex prefix
[0-9a-fA-F]+	One or more hexadecimal digits: 0-9 and both cases of a-f
$	Anchors the match to the end of the string

Detailed Explanation

What it does

This pattern requires a string to start with '0x' or '0X' and be followed by one or more valid hexadecimal digits (0-9, a-f, A-F). Strings without the prefix, or containing characters outside the hex digit set, are rejected.

Why it works

The literal segment 0[xX] mandates the prefix used by virtually every mainstream programming language for hex literals, while allowing either letter case. The character class [0-9a-fA-F] covers all sixteen valid hex digit symbols in both letter cases, and the + quantifier requires at least one digit after the prefix. Anchoring with ^ and $ prevents partial matches, such as a hex-looking substring embedded inside a longer string.

Common use cases

Validating hex literals in a code editor, linter, or template engine before evaluation
Checking user-entered memory addresses, color codes, or byte values that require the 0x prefix
Parsing configuration files or CLI flags that accept hexadecimal numeric input
Filtering log output for well-formed hex addresses versus malformed tokens

Edge cases

Mixed-case hex digits like '0xDeadBEEF' are accepted since both cases are included in the character class
A bare hex string without the prefix, such as '1A3F', is rejected by design; strip or require the prefix consistently upstream
The prefix alone with no following digits, '0x', is rejected because + requires at least one hex digit
Invalid characters like 'g' or 'G', which fall outside a-f/A-F, correctly cause a rejection, e.g. '0xGG'

Limitations

Requires the 0x/0X prefix unconditionally; if bare hex strings (e.g. from a color picker without '#') should also validate, use an alternation or a separate pattern
Does not enforce a specific digit count, so both very short and very long hex numbers are accepted; add a bounded quantifier for fixed-width values like 32-bit addresses
Does not validate that the resulting numeric value fits within a particular integer size without additional length constraints

Interactive Tester

Edit the pattern or text below — matching runs live in your browser.

Open in Playground →

0x1A3F 0Xff 0xdeadBEEF

3 matches

Test Cases

Editable — add your own inputs to see if they pass.

Input	Expected	Result
		Pass
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Language Variants

Production-ready examples in 12 languages.

const hexRegex = /^0[xX][0-9a-fA-F]+$/;
console.log(hexRegex.test('0x1A3F')); // true

Common Mistakes

Forgetting that hex digits are case-insensitive and only matching lowercase a-f, rejecting valid uppercase input like '0xFF'

Fix: Include both cases in the character class, [0-9a-fA-F], or apply the case-insensitive flag

Requiring the 0x prefix when the input source (e.g. a color picker) never includes it

Fix: Decide up front whether the prefix is mandatory and use a separate bare-hex pattern like ^[0-9a-fA-F]+$ if not

Confusing this pattern with one for CSS hex colors, which have fixed lengths of 3, 4, 6, or 8 digits and use a '#' prefix instead of '0x'

Fix: Use a dedicated hex-color pattern for CSS colors rather than reusing a generic numeric hex pattern

Performance Notes

The fixed-length prefix followed by a single bounded character class gives this pattern linear-time matching with no backtracking risk
Anchoring at both ends lets the engine reject non-hex strings as soon as the prefix check fails
For very large batches of hex validation, precompiling the regex once outside the loop avoids repeated compilation cost

Browser Compatibility

Engine	Supported	Notes
Chrome	Yes	—
Firefox	Yes	—
Safari	Yes	—
Edge	Yes	—
Node.js	Yes	—

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