Pattern

Regex for Octal number with 0o prefix

Modern octal literal: 0o755.

The Octal number with 0o prefix regex is ^0[oO][0-7]+$ — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

^0[oO][0-7]+$

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What it matches

0o755
0O644
0o0

What it doesn't match

755
0o8
0o

Notes & gotchas

Modern octal literal (0o prefix). Older C-style is just leading 0 (0755), which conflicts with decimal 0. Used in Python 3+, JavaScript ES6+.

Code in your language

Use the explainer's Code tab to generate ready-to-paste snippets in JavaScript, Python, Java, .NET, Go, Ruby, and PHP for this pattern.

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Token-by-token breakdown

Every part of the pattern, left to right:

Token	Meaning
`^`	start of string (or line in multiline mode)
`0`	literal text “0”
`[oO]`	any of: “o”, “O”
`[0-7]+`	one or more: any of: 0–7
`$`	end of string (or line in multiline mode)

About this pattern

Number patterns appear constantly in form validation, log parsing, and data extraction. Regex is great for matching the shape but limited for semantic checks (range validation, mathematical constraints).

Quick usage in different languages

This exact pattern — with the correct escaping and idioms for each language:

JavaScript: /^0[oO][0-7]+$/.test(value)
Python: re.match(r"^0[oO][0-7]+$", value)
Java: Pattern.compile("^0[oO][0-7]+$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"^0[oO][0-7]+$")
Go: regexp.MustCompile(`^0[oO][0-7]+$`).MatchString(value)
Ruby: /^0[oO][0-7]+$/.match?(value)
PHP: preg_match('~^0[oO][0-7]+$~', $value)

The explainer’s Code tab regenerates these for any pattern you paste, and the downloadable cheat sheet bundles the breakdown, all seven snippets, and the pitfalls below onto one printable page.

Common pitfalls

Anchored to the whole string. This pattern uses ^ and $, so it requires the entire input to match. To find it inside a longer text, drop the anchors and use the global (g) flag.
Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the octal number with 0o prefix against a source of truth (database, API, or checksum) where it matters.

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