Pattern

Regex for Linux username

Standard Linux/Unix username — POSIX rules.

The Linux username regex is ^[a-z_][a-z0-9_-]*[$]?$ — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

^[a-z_][a-z0-9_-]*[$]?$

Try in explainer → Download cheat sheet ↓

What it matches

root
ubuntu
www-data
user1
my_user

What it doesn't match

User1
1user
-bad
toolongxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Notes & gotchas

POSIX-compliant Linux usernames: lowercase letter or underscore first, then lowercase/digits/dashes/underscores. Optional trailing $ for Samba/NIS. Max 32 chars (not enforced here).

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)
`[a-z_]`	any of: lowercase letters, “_”
`[a-z0-9_-]*`	zero or more: any of: lowercase letters, digits, “_”, “-”
`[$]?`	optional (zero or one): any of: “$”
`$`	end of string (or line in multiline mode)

About this pattern

Developer-oriented patterns help with code analysis, log parsing, and data transformation. Use these in build scripts, CI tools, IDE search-and-replace, and code review utilities.

Quick usage in different languages

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

JavaScript: /^[a-z_][a-z0-9_-]*[$]?$/.test(value)
Python: re.match(r"^[a-z_][a-z0-9_-]*[$]?$", value)
Java: Pattern.compile("^[a-z_][a-z0-9_-]*[$]?$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"^[a-z_][a-z0-9_-]*[$]?$")
Go: regexp.MustCompile(`^[a-z_][a-z0-9_-]*[$]?$`).MatchString(value)
Ruby: /^[a-z_][a-z0-9_-]*[$]?$/.match?(value)
PHP: preg_match('~^[a-z_][a-z0-9_-]*[$]?$~', $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.
Lowercase only. Ranges like [a-z] won't match uppercase. Add A-Z (or the i flag) if uppercase input should be accepted.
Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the linux username against a source of truth (database, API, or checksum) where it matters.

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