Question

I need a regex which restricts an hour entry from 0-24 with the fraction in multiples of .25. Please keep in mind the following conditions must be met:

• When whole part is 0-23, the fractional part can be .0, .25, .5, or .75
• When whole part is 24, the fractional part can be .0
• When no whole part is present, the fractional part must be .0, .25, .5, or .75
• Empty string is not allowed

I am very confident that the following expression can be tightened up, but this is what is in place thus far:

(^(([0-9])|(1[0-9])|(2[0-3]))(\.((0*)|(250*)|(50*)|(750*)))?$)|(^24(\.0*)?$)|(^\.((0+)|(250*)|(50*)|(750*))$)

Update: The regex validation exists both on the client (javascript) and server (asp.net, c#).

Answer 1

And the reason why you absolutely, positively need to do this as a regular expression is why? Converting to a number and doing a few basic checks would be far easier.

Check if the number multiplied by 4 is a natural number <= 96, done.

Judging by your profile you’re probably using .NET. I assume you want this as a regular expression because you’re using a it for client side validation. I would seriously consider using a CustomValidator with a client side JavaScript.

Something along these lines for the JavaScript:

<script type="text/javascript">
function checkDuration(text)
{
    var tmp = parseFloat(text) * 4;
    return tmp >= 0 && tmp <= 96 && Math.ceil(tmp) == tmp;
}
</script>

Edit: I was slightly concerned about rounding errors when using floats, but it seems to work out okay for all possible cases.