Pattern

Regex for XML namespace declaration

xmlns or xmlns:prefix attribute.

The XML namespace declaration regex is xmlns(:[a-zA-Z][\w-]*)?="[^"]*" — copy it below, or open it in the explainer for a token-by-token breakdown.

The pattern

xmlns(:[a-zA-Z][\w-]*)?="[^"]*"

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What it matches

xmlns="http://www.w3.org/1999/xhtml"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

What it doesn't match

xmlns=
class="foo"
xmlns:1bad=""

Notes & gotchas

XML namespace declaration. Group 1 captures the prefix (with colon) if present. Default namespace has no prefix.

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
`xmlns`	literal text “xmlns”
`(`	start of a capturing group
`:`	literal text “:”
`[a-zA-Z]`	any of: lowercase letters, uppercase letters
`[\w-]*`	zero or more: any of: word chars (A–Z, a–z, 0–9, _), “-”
`)?`	end of group, optional (zero or one)
`="`	literal text “="”
`[^"]*`	zero or more: any character except “"”
`"`	literal text “"”

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: /xmlns(:[a-zA-Z][\w-]*)?="[^"]*"/.test(value)
Python: re.match(r'xmlns(:[a-zA-Z][\w-]*)?="[^"]*"', value)
Java: Pattern.compile("xmlns(:[a-zA-Z][\\w-]*)?=\"[^\"]*\"").matcher(value).matches()
C# / .NET: Regex.IsMatch(value, @"xmlns(:[a-zA-Z][\w-]*)?=""[^""]*""")
Go: regexp.MustCompile(`xmlns(:[a-zA-Z][\w-]*)?="[^"]*"`).MatchString(value)
Ruby: /xmlns(:[a-zA-Z][\w-]*)?="[^"]*"/.match?(value)
PHP: preg_match('~xmlns(:[a-zA-Z][\w-]*)?="[^"]*"~', $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.
Escape it correctly per language. In Java and JavaScript strings each backslash must be doubled (\\d); in Python, Go, and C# use raw/verbatim strings so the backslashes survive.
Validate beyond format. Matching the format doesn't guarantee the value is real. Confirm the xml namespace declaration against a source of truth (database, API, or checksum) where it matters.

Standards & sources

This pattern is based on the following authoritative specification(s) and issuing authorities. Formats can change — always confirm against the primary source.

W3C Namespaces in XML 1.0

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