Have part not be replaced in regex?

I'm using some regexes for swear filtering, but I have a problem. I need to look for ab[ct] and have it replaced by xyz but not replace the [ct]. I.e. it would be replaced by xyzc or xyzt. Any ideas? -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.