How to set a ignore area

[AkonXI]

I want to compare the difference between rich text and plain text and get each char's index in rich text. As the pic show ，HTML tags can affect the comparison results. so How could I set a regex such as /<(\S{1,}?)[^>]*>|<\/\1>/gms to ignore these tags but keep their positions in the string or treat HTML tags as a whole ？