Avoid matching regex to beginning of line

Question

I have come up with the following regex:

(.*\\\\\\\\documentclass.*)|(.*\\\\\\\\documentstyle.*)

This regex will be used to determine whether a document contains a \\documentclass or a \\documentstyle tag.

I would like to avoid matching a string starting with % such as this:

% To use with LaTeX, use \\documentstyle[psfig,...]{...}

How may I include a negative lookbehind of some sort to avoid matching to this string?

Answer 1

Anchor to the start of the line with ^ (with multiline mode), negative lookahead for % , and then alternate:

(?m)^(?!%)(?:.*\\document(?:class|style).*)