How do I create a regular expression to accept not more than 10 digits?

Question

How do I create a regular expression to accept not more than 10 digits?

thanks

Answer 1

Since you've asked "how", I'll try to explain step by step. Because you did not specify which regexp flavor you are using, so I'll provide examples in PCRE and two POSIX regexp variants.

For simple cases like this you should think of regular expression as of automation, accepting one character at a time, and saying whenever it is valid one (accepting the character) or not. And after each character you can specify quantifiers how many times it may appear, like (in PCRE dialect) * (zero or more times), + (one or more time) or {n,m} (from n to m times). Then the construction process becomes simple:

PRCE  | B.POSIX | E.POSIX   | Comments
------+---------+-----------+--------------------------------------------------
^     | ^       | ^         | Unless you are going to accept anything with 10
      |         |           | digits in it, you need to match start of data;
\d    | [0-9]   | [:digit:] | You need to match digits;
{1,10}| \{1,10\}| {1,10}    | You have to specify there is from 1 to 10 of them.
      |         |           | If you also wish to accept empty string — use 0;
$     | $       | $         | Then, data should end (unless see above).

So, the result is ^\d{1,10}$, ^[0-9]\{1,10}\$ or ^[:digit:]{1,10}$ respectively.

Answer 2

^\d{1,9}$

That will match anything from 1 digit to 9.

Since you didn't specify the regex flavor you're working with, that should get you where you need to be. If not, tell us which regex technology you're using.