Thanks for the good comments. My colleague was positively surprised with the number and quality of comments!

I think I will use the following approach:

• Read the articles mentioned (the PDF from Effective Perl Programming sure looks good!)
• Teach him the basics (., +, *, (/), character classes and the like) and what to use regexes for
• Use the regex evaluator together to gain proficiency
• Talk about basic pitfalls

Then, after a while we'll talk again about the traps he fell into :). By that time, I will also be able to see whether the more advanced concepts (look-ahead etc.) are really necessary for the work he needs to do.