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The sentence in question:
道の外側に出ちゃいけません！
While it is obvious what it means, I am wondering how precisely it is constructed grammatically.
The textbook way of expressing "must not do" using いけない is:
[て-Form] は いけない
So going by that I would construct it as follows:
出る → 出ちゃう → 出ちゃって → 出ちゃってはいけません
So I assume 「出ちゃいけません」 is simply a shortened form of this grammar.
Is that correct?
If yes, is this common?
If no, how is 「出ちゃいけません」 made up grammatically?
No, short for 出ちゃってはいけません would be 出ちゃっちゃいけません. ～ては itself contracts to ～ちゃ (compare ではない and じゃない), so 出ちゃいけません is a contracted form of 出てはいけません. This kind of contraction is extremely common.
