What is the function of と in this sentence?

Question

学生時代にもっと勉強すればよかったと、今ではとても残念だ。

This doesn't seem like the ～と...　pattern:

Expresses the idea that when ～ arises or happens, it will definitely lead to ...

Answer 1

In this context, と is used to refer to the preceding phrase, or 学生時代にもっと勉強すればよかった. This is because we can consider that と is a part of と思う and that 思う is omitted from the sentence. Indeed, by inserting 思う, we can paraphrase the sentence as follows:

(1) 学生時代にもっと勉強すればよかったと，今ではとても残念に思う．

(I should have studied more in my school years. Now I deeply regret that.)

Now, for a quick understanding of how と works, consider a simplified version of sentence (1):

(2) 学生時代にもっと勉強すればよかったと，今ではとても残念に思う．

(I think that I should have studied more in my school years.)

In sentence (2), と is indeed used to refer to the preceding phrase.

Next, for a further understanding of how と works, consider an expanded version of sentence (1):

(3) 学生時代にもっと勉強すればよかったと，今では学生時代にもっと勉強しなかったことをとても残念に思う．

(I should have studied more in my school years. Now I deeply regret that I did not study more in my school years.)

In sentence (3), と is essentially used to refer to the preceding phrase. This is because what I deeply regret now, or 学生時代にもっと勉強しなかったこと, can only be inferred from the phrase preceding と.