Pattern

Regex for camelCase identifier

Lowercase first word followed by capitalized words.

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

Open in explainer → Download cheat sheet ↓

Token-by-token breakdown

Every part of the pattern, left to right:

Token	Meaning
`^`	start of string (or line in multiline mode)
`[a-z]+`	one or more: any of: lowercase letters
`(`	start of a capturing group
`[A-Z]`	any of: uppercase letters
`[a-z0-9]*`	zero 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-Z][a-z0-9]*)*$/.test(value)
Python: re.match(r"^[a-z]+([A-Z][a-z0-9]*)*$", value)
Java: Pattern.compile("^[a-z]+([A-Z][a-z0-9]*)*$").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"^[a-z]+([A-Z][a-z0-9]*)*$")
Go: regexp.MustCompile(`^[a-z]+([A-Z][a-z0-9]*)*$`).MatchString(value)
Ruby: /^[a-z]+([A-Z][a-z0-9]*)*$/.match?(value)
PHP: preg_match('~^[a-z]+([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.
Case sensitivity. Letter ranges are case-sensitive — use the i flag if the input case can vary.
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 camelcase identifier against a source of truth (database, API, or checksum) where it matters.

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