Pattern

Regex for Hashtag

Social media hashtag — # followed by letters/digits/underscore.

The Hashtag regex is #[A-Za-z][A-Za-z0-9_]* — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

#[A-Za-z][A-Za-z0-9_]*

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What it matches

#regex
#code101
#WorldCup2026
#TIL

What it doesn't match

#
#123
# regex
##double

Notes & gotchas

Must start with a letter after the #. Numbers-only hashtags are conventionally not valid on most platforms. Use the /g flag to extract all hashtags from text.

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
`#`	literal text “#”
`[A-Za-z]`	any of: uppercase letters, lowercase letters
`[A-Za-z0-9_]*`	zero or more: any of: uppercase letters, lowercase letters, digits, “_”

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_]*/.test(value)
Python: re.match(r"#[A-Za-z][A-Za-z0-9_]*", value)
Java: Pattern.compile("#[A-Za-z][A-Za-z0-9_]*").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"#[A-Za-z][A-Za-z0-9_]*")
Go: regexp.MustCompile(`#[A-Za-z][A-Za-z0-9_]*`).MatchString(value)
Ruby: /#[A-Za-z][A-Za-z0-9_]*/.match?(value)
PHP: preg_match('~#[A-Za-z][A-Za-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

Not anchored. Without ^ and $ this can match a substring anywhere in the input — add anchors if you need the whole value to conform.
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 hashtag against a source of truth (database, API, or checksum) where it matters.

Related patterns

More patterns in the Identity & contact category: