Question

I am trying to compose a regular expression to match a numeric value expressed as a decimal multiple of .25 (ex. 1.25, 14.75).

// Must Match
1.0
1.25
1.250000
1.5
1.500
1.75
1.7500

// Must Not Match
1.2
1.46
1.501
1.99

So far I have the following expression: \d+(\.((0+)|(250*)|(50*)|(750*))). It works when I use online tooling like gskinner.com/regexr. When I use the expression in a validation attribute to seed my EntityFramework db, it produces validation errors:

[RegularExpression(@"^\d+(\.((0+)|(250*)|(50*)|(750*)))$", ErrorMessage = "Hours must be 15 minute increments expressed as decimals (ex. .0, .25, .5, .75)")]
public double Hours { get; set; }

Similar question (I am looking for a way to round the decimal portion of numbers up or down to the nearest .25, .5, .75, or whole number) but I need to use a regular expression to use the above data annotation.

Question:

1. Anyone see what’s wrong with my expression?

2. Bonus points if you can extend it to support whole numbers (ex. 4 or 4.25 but not 4. or 4.62)

Answer 1

To match such number use regex pattern

 (?!0\d)\d+(?:[.](?:25|5|75|0)0*)?(?!\d)

To validate input to be such number use regex pattern

 ^(?!0\d)\d+(?:[.](?:25|5|75|0)0*)?$

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In both cases, the very first part (?!0\d) is optional to disallow match/validate numbers with invalid leading zeros, such as 000003.250, when match would trim them and take just 3.250; validation would fail if this optional part is present in the regex.