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JavaScript

if (!/^[a-zA-Z][a-zA-Z0-9_]+(\.[a-zA-Z][a-zA-Z0-9_]*)+$/.test("be111.net")) {
        alert('Package name must look like: com.company.Name')
}

///run this and see its ok
///if (!/^[a-zA-Z0-9]+(\.[a-zA-Z0-9][a-zA-Z0-9_]*)+$/.test("be111.net")) {
///        alert('Package name must look like: com.company.Name')
///    }