What are some signs that a guy regrets sleeping with a girl?

The useful signal is a change in pattern, not one message. Look at how he behaved before the night together and compare it to the days after. If he used to start conversations and now only answers yours, that's a shift worth noticing. If he's still sending memes, still asking about your day, and still suggesting things to do, the night probably didn't change much for him.

Also look at what he does with plans. A guy who is interested gives you a day and a time or at least a clear next step. A guy pulling back tends to say we should hang out sometime, or keeps things open-ended so he doesn't have to commit. That vagueness is often more telling than a cold reply.

Your next message should be easy to answer and should tell him you're not waiting by the phone. Keep it about something real in your life or a question that's simple to reply to. The point is to give him a normal opening that costs him nothing to take.

Avoid anything that asks him to evaluate the night or define where you stand. If your message would be awkward to read aloud to a friend, or is mostly about how you feel about the silence, rewrite it around something outside the two of you. How he responds to a neutral message will tell you far more than any attempt to make him explain himself.

Timing shapes how a message reads. A late-night text after a hookup can look like an invitation to talk about feelings, which puts pressure on him. Send your message in the middle of a normal day, when you'd text anyone you're casually friendly with.

If he doesn't reply, wait a few days before any follow-up, and make that next message lighter, not heavier. If two more attempts over a couple of weeks get nothing, or he only answers when you bring up plans, you have your answer. Stop initiating and let him come to you. Continuing to send messages to a guy who has clearly stepped back only signals that you'll accept less.