Ain't in the context of "Eliza Doolittle"

In sociolinguistics, a register is a variety of language used for a particular purpose or particular communicative situation. For example, when speaking officially or in a public setting, an English speaker may be more likely to follow prescriptive norms for formal usage than in a casual setting, for example, by pronouncing words ending in -ing with a velar nasal instead of an alveolar nasal (e.g., walking rather than walkin), choosing words that are considered more formal, such as father vs. dad or child vs. kid, and refraining from using words considered nonstandard, such as ain't and y'all.

As with other types of language variation, there tends to be a spectrum of registers rather than a discrete set of obviously distinct varieties—numerous registers can be identified, with no clear boundaries between them. Discourse categorization is a complex problem, and even according to the general definition of language variation defined by use rather than user, there are cases where other kinds of language variation, such as regional or age dialect, overlap. Due to this complexity, scholarly consensus has not been reached for the definitions of terms such as register, field, or tenor; different scholars' definitions of these terms often contradict each other.