やってもた → やってしまった? What is the grammatical rule behind this?

Question

I recently saw 「やってもた」 used in a Japanese conversation, I don't remember the context exactly, it was something like：

久々にやってもた。

After asking my Japanese friend, it turns out to be some kind of Osaka-ben expression used　instead of 「やってしまった」. Where the　「も」 stands for 「しまう」. My friend couldn't give me another use of this so I'm here asking two questions:

1) Is my explanation 「やってもた」 for accurate?

2) Can we extract from this example a generic grammatical rule that can be applied to most situation?

I seek to learn Osaka-ben and I'm trying to replace my old habits, so I would be very grateful if you can provide some external resources about this or at least mention your sources.

Thank you very much.

Answer 1

1) It is "やってもうた" or "やってしもた".
2) The form "もうた" or "しもた" are shortened forms of "しもうた". The (auxiliary) verb "しまう" has a stem ending with the glide "w": "simaw-", and underwent different developments in Kansai and Kantoo regarding inflection. In Kantoo, the "w" was interpreted as a consonant, and was used to trigger gemination (a.k.a. [促]{そく}[音]{おん}[便]{びん}) in past tense:

simaw-ta → simatta

On the other hand, in Kansai, the "w" (back glide) was turned into "u" (high back vowel), and then underwent further change of vowels (k.a. ウ音便):

simaw-ta → simauta → simouta

If you are expecting external materials on these sound changes, you may want to study about ウ音便 and 促音便.