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What is this?

This is a collection of common Q&A. This is also a Community Wiki, so everyone is invited to participate in maintaining it.

Why is this?

regex is suffering from give me ze code type of questions and poor answers with no explanation. This reference is meant to provide links to quality Q&A.

What's the scope?

This reference is meant for the following languages: php, perl, javascript, python, ruby, java, .net.

This might be too broad, but these languages share the same syntax. For specific features there's the tag of the language behind it, example: