Beyond the Spelling Miracle: Investigating Substring Awareness in Character-Blind Language Models

Correctly identifying characters and substrings of words should be a basic but essential ability of any Language Model that aims to proficiently understand and produce language. Despite so, the majority of Pre-trained Language Models (PLMs) are "character-blind" and struggle in spelling tasks, although they still seem to acquire some character knowledge during pre-training, a phenomenon dubbed Spelling Miracle. To shed light on this phenomenon, we systematically evaluate a range of PLMs with different parameter sizes using a controlled binary substring identification task. Through a series of experiments, we propose the first comprehensive investigation on where, when, and how PLMs develop awareness of characters and substrings, with a particular linguistic focus on morphemic units such as prefixes, suffixes, and roots.