C#: Change NumberDecimalSeparator for Regex

Question

I'm parsing decimal values with a decimal separator using regex, that are in US format, like: 2.56

But my system's culture decimal separator is set to a comma, so when I use the decimal class \d, the regex fails to recognize "2.56" as a valid decimal number.

I would like to stick to the \d for the sake of readability and simplicity. Can I force it to recognize dot as a decimal separator?

When using float.TryParse(), I can enforce it to use dot as a decimal separator:

var usCulture = System.Globalization.CultureInfo.CreateSpecificCulture("en-US");
float.TryParse(text, System.Globalization.NumberStyles.Number, usCulture, out s )

Can I do something similar for regex?

I tried changing current thread's culture:

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
Thread.CurrentThread.CurrentUICulture = new CultureInfo("en-US");

but it doesn't seem to affect the regex, it still expects comma as a decimal separator.

`\d` matches a digit, not a "number", and does not match any decimal separators. — Wiktor Stribiżew, Apr 17 '18 at 08:07
I misread the documentation then. There is no built-in class that matches a decimal number, it must be matched manually? — Yekoor, Apr 17 '18 at 08:12
@Yekoor, yes you have to put the pattern manually, something like `^-?[0-9]+(\.[0-9]+)?$` — Dmitry Bychenko, Apr 17 '18 at 08:13
See https://stackoverflow.com/questions/9107673/validate-float-number-using-regex-in-c-sharp — Wiktor Stribiżew, Apr 17 '18 at 08:14
See https://stackoverflow.com/questions/273141/regex-for-numbers-only — Wiktor Stribiżew, Apr 17 '18 at 08:14
See https://stackoverflow.com/questions/968825/regular-expression-for-decimal-number — Wiktor Stribiżew, Apr 17 '18 at 08:15
And after the match - what are you doing with those numbers? — Evk, Apr 17 '18 at 08:41

score 1 · Answer 1 · answered Apr 17 '18 at 08:12

I would like to stick to the \d for the sake of readability and simplicity. Can I force it to recognize dot as a decimal separator?

No, that is because \d is shorthand for [0-9]. It doesn't match for anything else, including separators.

If you want to match decimals, you need something like this:

-?\d+(\.\d+)?

Demo

Dmitry Bychenko · Answer 2 · 2018-04-17T11:19:20.130

Well, \d is a any unicode digit, not a floating point number. If you insist on Regular Expressions you have to construct pattern manually; in the simplest case (no exponential part, no thousand separator, arbitrary length etc.) it can be

  string escapedSeparator = Regex.Escape(
    CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator);

  string pattern = $"^-?[0-9]+({escapedSeparator}[0-9]+)?$";

  if (Regex.IsMatch("-123.456", pattern)) {
    ...
  }

I really doubt if it's better than double.TryParse(...) which has been specially designed for this purpose.

C#: Change NumberDecimalSeparator for Regex

2 Answers2