这不是普通话，而是上海话。

This isn't Mandarin, it's Shanghainese.

Source: Tatoeba

Sentence Composition

=1 这不 zhèbu (coll.) As a matter of fact, ... (used to introduce evidence for what one has just asserted)
=1 这不 zhèbù connects two clauses, emphasising how the following clause substantiates the preceding clause
=2 是 shì
  1. to be (followed by substantives only)
  2. correct; right; true
  3. (respectful acknowledgement of a command) very well
  4. (adverb for emphatic assertion)
=2 是 shì variant of 是 shì
3 普通话 pǔtōnghuà
  1. Mandarin (common language)
  2. Putonghua (common speech of the Chinese language)
  3. ordinary speech
4 而是 érshì rather
=5 上海话 Shànghǎihuà
  1. Shanghainese
  2. Shanghai dialect
=5 上海话 shànghǎihuà The Shanghainese language