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^(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]{4,20}(?<![_.])$ | |
└──┬───┘└──────┬────┘└─────────┬───────┘└──┬────┘ | |
│ │ │ no _ or . at the end | |
│ │ allowed chars & length | |
│ no __ or _. or ._ or .. inside | |
no _ or . at the beginning |
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