From Teflpedia

Ambiregularity is a property of a word that has both attested regular and irregular forms.

English has some ambiregular words, in particular:

  • Ambiregular verbs, have a regular preterite and past participle formed by inflection with -ed, and either or both of (1) an irregular preterite form or (2) an irregular past participle.
  • Ambiregular nouns generally have two plurals; a regular plural formed with inflection with plural -s and an irregular plural formed in another way.
  • Ambiregular pronunciation has one pronunciation that is regular, and one that doesn't follow the spelling, e.g. "grass" can be pronounced /græs/ per the spelling or (irregularly) /grɑ:s/ in a southern British accent.
  • Ambiregular spelling has two spellings one following the pronunciation, one not. e.g. "jail" and "gaol" (both /ʤeɪl/).

The good news for language learners is that they only have to learn to produce the regular version of the word; but they may need to recognise the irregular version.