"00000000000000000000000000000" matches Regex "^[1-9]|0$"

Question

In .Net4.5, I find that the result of

System.Text.RegularExpressions.Regex.IsMatch(
    "00000000000000000000000000000", "^[1-9]|0$")

is true.

The result I expect is false. I don't know why. Can you help me?

Update: In the beginning, I was validating the regular expression ^-?[1-9]\d*|0$ which is used to match integer found on the internet and I find that the string with multiple 0 matches the regular expression.

Answer 1

The issue is the alternator's binding behavior. By default (i.e. without using grouping), an expression containing an alternator (|) will match either the value to the left of the alternator, or the value to the right.

So in your expression, you're matching either one of these:

• ^[1-9]
• 0$

Your call to the IsMatch method returns true because the second of those two option matches the string 00000000000000000000000000000.

To restrict the alternator's binding to a specific part of your expression, you need to group using parentheses, as follows:

^([1-9]|0)$

---

Putting all this together, a strict expression to validate integers, disallowing leading zeroes and negative zero, could look like this:

^(-?[1-9][0-9]*|0)$