Pattern

Regex for snake_case identifier

Lowercase words joined with underscores.

The snake_case identifier regex is ^[a-z][a-z0-9]*(_[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]*(_[a-z0-9]+)*$

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What it matches

user_name
first_name
my_variable_2

What it doesn't match

User_Name
user__name
_leading
trailing_

Notes & gotchas

No consecutive underscores, no leading/trailing underscores. Used for Python variables and Ruby methods.

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
`(`	start of a capturing group
`_`	literal text “_”
`[a-z0-9]+`	one or more: any of: lowercase letters, digits
`)*`	end of group, zero or more
`$`	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, 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]*(_[a-z0-9]+)*$/.test(value)
Python: re.match(r"^[a-z][a-z0-9]*(_[a-z0-9]+)*$", value)
Java: Pattern.compile("^[a-z][a-z0-9]*(_[a-z0-9]+)*$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"^[a-z][a-z0-9]*(_[a-z0-9]+)*$")
Go: regexp.MustCompile(`^[a-z][a-z0-9]*(_[a-z0-9]+)*$`).MatchString(value)
Ruby: /^[a-z][a-z0-9]*(_[a-z0-9]+)*$/.match?(value)
PHP: preg_match('~^[a-z][a-z0-9]*(_[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.
Watch for backtracking. This pattern nests quantifiers; on adversarial input that can cause exponential backtracking (ReDoS). Test with long non-matching strings, or use an RE2-based engine.
Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the snake_case identifier against a source of truth (database, API, or checksum) where it matters.