Hi Jabby
In the regex:
_(.+?)_
the brackets designate a "backreference" - essentially marking whatever string is matched by the tokens inside the brackets, so that that string can be used as part of the replacement string. You might have several of these in the search string - the first "backreference" would be referenced in the replacement string by "\1", the second by "\2", etc.
In this instance, the regex is looking to match any number of characters ("." = any character, "+" = from 1 to unlimited number, "?" = as few as possible, expanding as needed) between two underscore characters. The brackets designate the text found between the underscore characters as a backreference, for use in the replace string, and since that's the first backreference specified in the regex, it will be referenced as "\1" in the replace string.
The replace string:
<i>\1</i>
specifies a string consisting of "<i>", followed by the text matched in backreference number 1, followed by "</i>".
Someone else might be able to suggest a good online tutorial, Jabby. I'm usually good at understanding technical stuff, but I have to admit I struggled to pick up regex from reading stuff online. What I've found invaluable, though, is a small program called
RegexBuddy from JGSoft (the same guys who produce EditPad Pro) - it provides tools to help build regular expressions, and the way you build and view them helps you learn the syntax. It's not free, but it's not expensive either (30 Euros), so might be worth considering.