
Never seen such a form in Mandarin. Is it frequent?

I orally hear it quite often, in the Jiangsu province

Interesting. Can it be dialectic?

well i don't know, nickyeow is from Hong Kong (or at least this part of China), which is several thousand kilometers from my place, so if he also say like this, that's maybe more than only dialectal? Maybe very colliqial ?

Mandarin isn't my mother tongue so I'm not really in a position to judge whether expressions are colloquial or not... but I can guarantee you I've come across the form quite a number of times in Chinese publications.
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