Saira,
I realise that it is in a companion forum to this ine, but the sticky note at http://regexadvice.com/forums/thread/43218.aspx will tell you the basic information that you need to provide us so that we can help you.
Having said that, the 'ignore case' option is fairly standard on most regex engines and will cause upper and lower case character to be treated the same.
Also most regex engines provide a 'replace' function that will make the changes that you are after.
However, if the 'font' tags can be nested then things get very different depending on the regex engine you are using. Also, I'm assuming that there are multiple occurrences of this within your text, so ca nwe see an example of the real text you are working on?
Also, which parts of the tag are constant and which are variable? Can there be other attributes within the opening tag? Is the colour always 'red'?
Susan