MacOS – Do any OS X apps ease Search and Replace across multiple files

I'm looking for an OS X tool which allows me to search and replace a search string in multiple files. The requirements are the following:

• Search and Replace
• Search by regular expression
• Search across multiple files
• Search across nested directory structures
• Replace expression can use values from the search (Regular Expression)
• The tool has an easy to use user interface
• Preferably free

I know that some text editors like BBEdit have this functionality, but I would prefer a lightweight tool that doesn't require me to open the files for editing first.

If there aren't any tools to match these requirements, I would also consider a command line alternative. I know that perl and sed support something like this – a solution that I can define either as a Bash alias or function would be welcome in this case.