particle は - Contraction of や, the variant of は

In an answer to this question, Tsuyoshi Ito correctly points out that you can sometimes change the consonant や, a variant of は, into a glide but sometimes you cannot.

聞きやしない
聞きゃ(ー)しない

　大きすぎやしない
* 大きすぎゃ(ー)しない

When can you do it, when can you not?

Answer

I think that the rule is as follows: ignoring -suru verbs and special verb 来る, which cannot be contracted, verbs whose stem ends with a consonant other than -w (ワア行以外の五段活用の動詞) can be contracted, and the other verbs cannot be contracted. This is at least consistent with the given examples and some other examples which I came up with.

• 聞く (kik-u): 聞きや (kik-i-ya) → 聞きゃ (kik-ya) is acceptable


• 大きすぎる (ookisugi-ru): 大きすぎや (ookisugi-ya) → 大きすぎゃ (ookisug-ya) is not acceptable


• 笑う (< waraw-u): 笑いや (< waraw-i-ya) → ??? (waraw-ya) is not acceptable

Also this rule seems to make sense: the vowel “i” of the bold き in 聞きやしない is not part of a verb stem and is inserted just to attach the particle や, and therefore it can be omitted. The vowel “i” of ぎ in 大きすぎやしない is part of a verb stem and therefore it cannot be omitted. In the case of verbs whose stem ends with -w such as 笑う (< waraw-u), 笑いや (< waraw-i-ya) cannot be contracted to waraw-ya simply because we cannot say “wya.”

Added: After posting this answer, I have realized that some people use the form [笑]{わら}やしない. I think that this is at least more acceptable than 大きすぎゃしない, and that perhaps it is as acceptable as other examples such as 聞きゃしない. If this is the case, the rule has to be modified: it is acceptable to contract verbs whose stem ends with -w, in which case -w-i-ya becomes just -ya. This might be a result of two changes: first -w-i-ya becomes -w-ya as in other verbs, and then -w-ya automatically becomes -ya just as -w-u becomes -u.