Regular expression for decimals that are accepted in decimal.Parse

Question

The desired result is a pattern for positive inputs that doesn't throw an exception in decimal.Parse and doesn't accept the 01-model-like inputs.

Valid inputs:

1
1.2
.2
1.

Invalid inputs:

01
01.2
.
1.B
A.2
A
A.

I liked the pattern in this answer (-?(0|([1-9]\d*))(\.\d+)?); but somehow (as mentioned in the comment), it accepts X.2 (only with Regex.IsMatch) and negative decimals, and rejects 1.; so I modified it to /((0|([1-9]\d*))(\.\d*)?)|(\.\d+)/g, and it worked perfectly in regexr.com and also RegExr v1; but I got no luck with Regex.IsMatch.

Any suggestions? and why doesn't the pattern work with Regex.IsMatch while it works somewhere else?

Do the answers below answer your question? If yes, please accept the one that works. — Wiktor Stribiżew, Apr 02 '16 at 18:27
Try [`Regex.IsMatch(str, @"^(?!0)(?:\d+\.?\d*|\.\d+)$")`](http://ideone.com/hTSHXp). See regex [demo](https://regex101.com/r/yG4bQ1/1). — Wiktor Stribiżew, Apr 02 '16 at 20:13
@WiktorStribiżew; I apologize for being late. The pattern works perfectly; if you post an answer, I'd up-vote it (best answer already chosen). — Aly Elhaddad, Apr 02 '16 at 22:44

score 2 · Accepted Answer · answered Apr 02 '16 at 14:35

This passes all your tests:

var reg = new Regex("^(([1-9]*[0-9](\\.|(\\.[0-9]*)?))|(\\.[0-9]+))$");

(reg.IsMatch("1") == true).Dump();
(reg.IsMatch("1.2") == true).Dump();
(reg.IsMatch(".2") == true).Dump();
(reg.IsMatch("1.") == true).Dump();

(reg.IsMatch("01") == false).Dump();
(reg.IsMatch("01.2") == false).Dump();
(reg.IsMatch(".") == false).Dump();
(reg.IsMatch("1.B") == false).Dump();
(reg.IsMatch("A.2") == false).Dump();
(reg.IsMatch("A") == false).Dump();
(reg.IsMatch("A.") == false).Dump();

Explanation:

We try to capture as many 1-9 numbers as we can. This excludes leading 0s. We then allow any number before the decimal point.

Then we have three cases: No decimal point, a decimal point, a decimal point with numbers following it.

Otherwise, if we start with a decimal point, we allow any number, but at least one after it. This excludes .

score 0 · Answer 2 · edited May 23 '17 at 12:07

0

You've to remove '/' character at the beginning. Since it's the syntax of regular expression to specify the start of regex in Javascript/ECMAScript but not c#.(Refer: What does the forward slash mean within a JavaScript regular expression?) Thus, final regex is

((0|([1-9]\d*))(\.\d*)?)|(\.\d+)/g

edited May 23 '17 at 12:07

Community

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answered Apr 02 '16 at 14:41

M.S.

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Well! it worked with the valid inputs; but with the invalid ones, it did accept: `01`, `01.2`, `1.B` and `A.2`. – Aly Elhaddad Apr 02 '16 at 22:50

Redu · Answer 3 · 2016-04-02T17:16:44.773

0

I guess this simple regex should do the job perfectly well /-?(0?\.\d+|[1-9]\d*\.?\d*)/g

You haven't asked for it but it also handles the negatives. If not needed just delete the -? at the beginning and make it even more simpler.

edited Apr 02 '16 at 17:16

answered Apr 02 '16 at 16:56

Redu

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I removed the `-?`; it returned `false` with all inputs mentioned in the question. – Aly Elhaddad Apr 02 '16 at 22:46
Please check it at http://regexr.com/3d4va with your inputs... With or without using `-?` in the front – Redu Apr 02 '16 at 23:13
The pattern mentioned in your comment works fine although it's different from that one in your answer; however, thanks for your effort. – Aly Elhaddad Apr 02 '16 at 23:48

Regular expression for decimals that are accepted in decimal.Parse

3 Answers3