How would you build a regex to change the src here?

<img alt="></(/>" src="/img_handler?alpha=>0.9;name=fish.png" />
[download]

Even if you managed to get this right, the likelihood is very high that your pattern could be broken easily. Don't just blindly look for strings in your HTML.

Write an actual parser, if you have a lot of time to spare. Otherwise, use one of the existing ones (see Ovid's reply) and get on with your life.

Makeshifts last the longest.