Regular expression to match a string that contains only numbers, not letters

Question

My code is currently using the following Regex expression which matches on numbers:

Regex numberExpression = new Regex(@"(?<Number>\d+)");

This current works fine for input strings like "1", "100", "1a", "a1", etc....

But I want to change it so it does NOT match when the input string contains a letter, so "1", "100" would match, but "1a", "a1", would not.

Can anyone help, I know this is a simple regular expression question but I can't get my head around the forward and backward looking. I have tried:

Regex numberExpression = new Regex(@"(?<Number>^![a-zA-Z]\d+![a-zA-Z])");

but that didn't work, and fails to match any of the above input.

You clearly haven't put enough efforts/research. Please read about [anchors](http://www.regular-expressions.info/anchors.html). — HamZa, Jul 25 '13 at 11:24
@HamZa There is a lot worse questions. This is quite a good one. — It'sNotALie., Jul 25 '13 at 11:24
@newStackExchangeInstance I know. Today I voted to close about 40 regex questions, still it does not show efforts. Spending 10 minutes (at most 1 hour) reading a basic tutorial should get you to the solution. — HamZa, Jul 25 '13 at 11:26
@HamZa I have looked at anchors, on that site in fact, but it's the negation that is throwing me — stackman, Jul 25 '13 at 11:27
@stackman Check this [answer](http://stackoverflow.com/a/2759417) — HamZa, Jul 25 '13 at 11:32

score 9 · Answer 1 · answered Jul 25 '13 at 11:23

9

Regex is overkill. Try this:

input.All(char.IsDigit);

answered Jul 25 '13 at 11:23

It'sNotALie.

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score 5 · Accepted Answer · answered Jul 25 '13 at 11:31

5

You are trying to do it the hard way, by looking for a numeric substring of the input, and then looking to see that there isn't anything before or after that substring.

The easy way to do it is to force the regular expression to either match the entire input string or nothing:

Regex numberExpression = new Regex(@"^\d+$");

where "^" means "beginning of line" and "$" means "end of line".

answered Jul 25 '13 at 11:31

mbeckish

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@stackman: Are you OK with the fact that this regex will also accept `"०१२۱۲۳"`? – Tim Pietzcker Jul 25 '13 at 12:32

Regular expression to match a string that contains only numbers, not letters

2 Answers2