1. He should have thought more carefully.
2. He regrets that he didn't think more carefully.
3. I regret that he didn't think more carefully.
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Does #1 mean #2 or #3?
#1 means #2
Aghedo is right. Do you see why #3 doesn't mean the same as either of the others?
4. You should have come earlier.
5. I'm sorry you didn't come earlier.
(Doesn't #4 mean #5?)
4 and 5 are similar in meaning, but not identical.
To determine whether sentence #1 means #2 or #3, we need to analyze the pronoun "he" and its reference.
In sentence #1, the pronoun "he" refers to someone who should have thought more carefully. Without any additional context, it is impossible to determine whether "he" refers to the speaker or another person.
Therefore, sentence #1 could mean either #2 or #3, depending on who "he" refers to. If "he" refers to the speaker, then sentence #1 means #2. If "he" refers to someone other than the speaker, then sentence #1 means #3.