EasyNumbers3 min read

Octal Number Regex

Validates a base-8 (octal) number, accepting digits 0-7 with an optional case-insensitive '0o' prefix as used by modern language literals.

#octal#numbers#base-8#validation#regex#string-matching

Regex Pattern

^(0o)?[0-7]+$

Default flags: i

Pattern Breakdown

Hover over a token to see what it does.

^(0o)?[0-7]+$

Token	Meaning
^	Anchors the match to the start of the string
(0o)?	Optional literal '0o' prefix (case-insensitive due to the i flag), matching modern octal literal syntax like 0o17
[0-7]+	One or more digits, each restricted to the octal range 0 through 7
$	Anchors the match to the end of the string

Detailed Explanation

What it does

This pattern validates that a string is a well-formed octal number: an optional '0o' or '0O' prefix followed by one or more digits from 0 to 7. It rejects any digit 8 or 9, letters other than the prefix 'o', and empty input.

Why it works

The optional group (0o)? matches languages like JavaScript, Python, and Rust that spell octal literals with a leading 0o, while making the prefix optional also allows bare digit strings like '17' to validate as octal. The i flag makes the 'o' in the prefix match both upper and lower case. The character class [0-7] excludes 8 and 9, which are not valid octal digits, and the + quantifier requires at least one digit. Anchoring with ^ and $ ensures the whole string, not a substring, is checked.

Common use cases

Validating octal literals in a code editor or linter for languages that support 0o-prefixed numbers
Checking Unix file permission strings before passing them to a chmod-style API
Parsing octal escape sequences or configuration values that use base-8 notation
Building a base converter tool that accepts octal input in either prefixed or bare form

Edge cases

A bare digit string like '755' (common for Unix permissions) is accepted even without the 0o prefix
Mixed case prefixes like '0O17' are accepted because of the case-insensitive i flag
Digits 8 and 9, as in '789', are correctly rejected since they fall outside the octal range
An empty string, or just the prefix '0o' with no following digits, is rejected

Limitations

Does not distinguish between the legacy C-style leading-zero octal notation (e.g. '0755') and a plain decimal string that happens to start with 0, since both look like bare octal digits here
Does not enforce a maximum value or bit width; add a bounded length if a fixed-size octal value (e.g. 3-digit permissions) is required
Treats the prefix as fully optional, so callers needing to require the 0o prefix should change (0o)? to 0o

Interactive Tester

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Open in Playground →

0o17 17 0O755

3 matches· capture groups: 1

Test Cases

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

Input	Expected	Result
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Language Variants

Production-ready examples in 12 languages.

const octalRegex = /^(0o)?[0-7]+$/i;
console.log(octalRegex.test('0o17')); // true

Common Mistakes

Using the decimal digit class [0-9] instead of [0-7], which lets invalid octal digits 8 and 9 slip through

Fix: Restrict the character class to [0-7] since octal only has eight valid digits

Requiring the '0o' prefix unconditionally, which rejects legitimate bare octal strings like Unix permission codes '755'

Fix: Make the prefix optional with (0o)? unless the use case specifically requires the modern literal syntax

Forgetting the case-insensitive flag and missing valid uppercase-prefixed literals like '0O17'

Fix: Add the i flag (or its language equivalent) so both '0o' and '0O' are accepted

Performance Notes

The optional prefix group and bounded character class keep this pattern linear-time with no backtracking risk
Because [0-7] is a small, non-overlapping character class, the engine can reject invalid digits in constant time per character
Precompiling the regex once and reusing it avoids repeated compilation overhead in hot loops such as bulk permission-string validation

Browser Compatibility

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

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