How do I match a string up to the first comma (if present) with a Ruby regexp

I'm struggling to get a regexp (in Ruby) that will provide the following

"one, two" -> "one"
"one, two, three" -> "one"
"one two three" -> "one two three"

I want to match any characters up until the first comma in a string. If there are no commas I want the entire string to be matched. My best effort so far is

/.*(?=,)?/

This produces the following output from the above examples

"one, two" -> "one"
"one, two, three" -> "one, two"
"one two three" -> "one two three"

Close but no cigar. Can anyone help?