Matching numbers at the beginning of a string?

I was expecting a true while trying to match 6 numbers from the beginning of string, instead i am getting a false.
What is wrong with this regular expression?

utop # Str.string_match (Str.regexp "^\\d{6}") "122323" 0 ;;
- : bool = false  
 

 The following works, but  that is not  exactly i am looking for.
 

 utop # Str.string_match (Str.regexp "^[0-9]+") "122323" 0 ;;
- : bool = true 

 

 Cheers.
 - jo