I have files with sequences of conversations where speakers are tagged. The format of my files is:
I am John
And I am Lisa
Here is a regex you can use to match what you describe:
And a small demo showing that it works: https://regex101.com/r/iW8vS5/1
However, as both kchinger and owler mentioned, regex probably isn't the best way to do this. A regex solution would likely be significantly slower than a small snippet of code for any long document.