Double Regex gives back the wrong number

Question

I made a Regex which filters for double values.

For example one TextBox contains volume and I use this regex for

double ConvertToDouble(String input)
{
    // Matches the first numebr with or without leading minus.
    Match match = Regex.Match(input, "[+-]?\\d*\\.?\\d+");

    if (match.Success)
    { 
        return Double.Parse(match.Value);
    }
    return 0; // Or any other default value.
}

Somehow this returns 0 when I enter 0,5 into the TextBox.

`0,5` contains *comma* `,` when you match for *dot* - `\\.`; that's why you have no match for `[+-]?\\d*\\.?` but for the *last* `\\d+`. I suggest adding *anchors* `^`, `$` - `"^\\s*[+-]?\\d*\\.?\\d+\\s*$"` — Dmitry Bychenko, Oct 01 '19 at 07:57
According to [RegExr](https://regexr.com/4m0cl), there will be two matches - `0` and `5`. If, as @DmitryBychenko suggests, you change the comma to a dot, there will be a single match of `0,5` — Diado, Oct 01 '19 at 07:59
You could also consider typing `@"string with \"` to avoid escaping the `\` character — Cleptus, Oct 01 '19 at 07:59
`1,023,024.37 m^3` might pose problems - if you get numbers like that. Same for `1.203,74 m^3` — Patrick Artner, Oct 01 '19 at 08:02
Does not double.tryparse Crash the program if there is no Input — Explorer2501, Oct 01 '19 at 08:02

score 6 · Accepted Answer · answered Oct 01 '19 at 08:01

6

I suggest using double.TryParse and get rid of regex at all:

// static : we don't use "this" in the method
static double ConvertToDouble(String input)
{
    // if we can parse input...
    if (double.TryParse(input, out double result))
        return result; // ...return the result of the parsing
    else
        return 0.0;    // otherwise 0.0 or any default value
}

answered Oct 01 '19 at 08:01

Dmitry Bychenko

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but if for example my TextBox is empty and i try to double.tryparse does it not Crash?? Double.parse crashes for sure is there a difference ? – Explorer2501 Oct 01 '19 at 08:07
1

@Explorer2501 The Try in TryParse means that it won't throw an exception on empty or invalid input. That's the difference between Parse and TryParse. – ckuri Oct 01 '19 at 08:08
@Explorer2501: If `input` is empty, then `double.TryParse` fail to parse it and return `false` and we on turn return `0.0` – Dmitry Bychenko Oct 01 '19 at 08:09

score 0 · Answer 2 · answered Oct 01 '19 at 07:59

Your regex is currently only matching against decimals specified with dot (.) and not a comma (,).

You could modify the regex to accept either using:

[+-]?\d*[\,\.]?\d+

In your code:

Match match = Regex.Match(input, @"[+-]?\d*[\.\,]?\d+");

This would match both 0,5 and 0.5.

Double Regex gives back the wrong number

2 Answers2