I create a text field for adding tags separated by commas. (e.g. php, jquery, js, ruby on rails) The field is like the one on stakoverflow where you add tags for posts.
OK, there are a few problems here:
The line you posted will result in a syntax error in PHP. Since you "tested it", it means this is not really the code you used. Please post the actual code.
About the regex, it's a good first try, but you forgot the numbers:
/^[a-z,0-9 ]+$/i