Pattern

Regex for Username

Alphanumeric username, 3–20 characters, starts with a letter.

The Username regex is ^[a-zA-Z][a-zA-Z0-9_-]{2,19}$ — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

^[a-zA-Z][a-zA-Z0-9_-]{2,19}$

Try in explainer → Download cheat sheet ↓

What it matches

alice
user_123
john-doe
BobSmith

What it doesn't match

12alice
_user
a
this_username_is_way_too_long

Notes & gotchas

Starts with a letter; letters/digits/underscore/hyphen allowed. Adjust the length range to fit your signup policy.

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.

Open in explainer →

Token-by-token breakdown

Every part of the pattern, left to right:

Token	Meaning
`^`	start of string (or line in multiline mode)
`[a-zA-Z]`	any of: lowercase letters, uppercase letters
`[a-zA-Z0-9_-]{2,19}`	between 2 and 19 times: any of: lowercase letters, uppercase letters, digits, “_”, “-”
`$`	end of string (or line in multiline mode)

About this pattern

Identifying and validating user identity (names, contact info, IDs) is one of the most common reasons developers reach for regex. The pattern below handles the format check; for full validation always confirm against a source of truth (database, API, or document).

Quick usage in different languages

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

JavaScript: /^[a-zA-Z][a-zA-Z0-9_-]{2,19}$/.test(value)
Python: re.match(r"^[a-zA-Z][a-zA-Z0-9_-]{2,19}$", value)
Java: Pattern.compile("^[a-zA-Z][a-zA-Z0-9_-]{2,19}$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"^[a-zA-Z][a-zA-Z0-9_-]{2,19}$")
Go: regexp.MustCompile(`^[a-zA-Z][a-zA-Z0-9_-]{2,19}$`).MatchString(value)
Ruby: /^[a-zA-Z][a-zA-Z0-9_-]{2,19}$/.match?(value)
PHP: preg_match('~^[a-zA-Z][a-zA-Z0-9_-]{2,19}$~', $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.
ASCII letters only. [a-zA-Z] excludes accented and non-Latin letters (é, ü, ß, ñ, and non-Latin scripts). For international input use Unicode properties like \p{L} with the u flag.
Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the username against a source of truth (database, API, or checksum) where it matters.

Related patterns

More patterns in the Identity & contact category: