Regex to match "www" or nothing

I have the following URLs:

www.localho开发者_运维技巧st.com
localhost.com
test.localhost.com

How would I match "www" or nothing in a Regex?

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(?:www)?

should match www or nothing.

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/^www/ alows you to check if subjects starts with www.

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If I understand your question correctly, you need to use a lookahead, like this:

/^www(?=\.localhost.com)/

This will match www if and only if it's followed by .localhost.com.

If that's not what you're trying to do, please clarify.

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www.+? It matches all string that has prefix as www