Regular expressions are a very naive way of detecting calls to 'require'. For correctness you're better off recursively walking the AST.
I've successfully used Detective in a couple of my personal projects to find all require statements.
Relevant issue on Detective: https://github.com/substack/node-detective/issues/8
The AST? What AST is that? You don't get access to an AST.
Tools like acorn[1] and Esprima[2] can parse the JavaScript source and output an ESTree compliant syntax tree.
Then you can traverse it and modify it like any other AST.
[1] https://github.com/ternjs/acorn [2] http://esprima.org/ [3] https://github.com/estree/estree