I want to validate a string only if it contains '0-9' chars with length between 7 and 9.
What I have is [0-9]{7,9} but this matches a string of ten chars too, which I don't want.
If you want to find 7-9 digit numbers inside a larger string, you can use a negative lookbehind and lookahead to verify the match isn't preceded or followed by a digit
(?<![0-9])[0-9]{7,9}(?![0-9])
This breaks down as
(?<![0-9]) ensure next match is not preceded by a digit[0-9]{7,9} match 7-9 digits as you require(?![0-9]) ensure next char is not a digitIf you simply want to ensure the entire string is a 7-9 digit number, anchor the match to the start and end with ^ and $
^[0-9]{7,9}$