6 digits regular expression

Solution 1:

You can use range quantifier {min,max} to specify minimum of 1 digit and maximum of 6 digits as:

^[0-9]{1,6}$

Explanation:

^     : Start anchor
[0-9] : Character class to match one of the 10 digits
{1,6} : Range quantifier. Minimum 1 repetition and maximum 6.
$     : End anchor

Why did your regex not work ?

You were almost close on the regex:

^[0-9][0-9]\?[0-9]\?[0-9]\?[0-9]\?[0-9]\?$

Since you had escaped the ? by preceding it with the \, the ? was no more acting as a regex meta-character ( for 0 or 1 repetitions) but was being treated literally.

To fix it just remove the \ and you are there.

See it on rubular.

The quantifier based regex is shorter, more readable and can easily be extended to any number of digits.

Your second regex:

^[0-999999]$

is equivalent to:

^[0-9]$

which matches strings with exactly one digit. They are equivalent because a character class [aaaab] is same as [ab].

Solution 2:

  ^\d{1,6}$

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