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I'm using this regex to match some strings:

^([^\s](-)?(\d+)?(\.)?(\d+)?)$/

I'm confusing about why it's permitted to enter two dots, like ..

What I understand is that only allowed to put 1 dash or none (-)? Any digits with no limit or none (\d+)? One dot or none (\.)?

Why is allowed to put .. or even .4.6?

Testing done in http://www.regextester.com/

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  • What are the strings you want to match and which are that you do not want to match? [^\n] is a negated character class that matches .. Add it to [^\n.], and it will not match. Commented Aug 11, 2015 at 14:17

3 Answers 3

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[^\s] means anything that is not a whitespace. This includes dots. Trying to match .. will get you:

  1. [^\s] matches .
  2. (-)? doesn't match
  3. (\d+)? doesn't match
  4. (\.)? matches .
  5. (\d+)? doesn't match

I'll assume you wanted to match numbers (possibly negative/floating):

^-?\d+(\.\d+)?$
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2 Comments

Exactly right. Regexplained might give some clarity
Omg, how can I didn't notice it. Thank you!
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^([^\s](-)?(\d+)?(\.)?(\d+)?)$/

Created with RegexBuddy

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Yeah, the reputation goes to the tool :)
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As I mentioned in my comment, [^\n] is a negated character class that matches .. and as there is another (\.)? pattern, the regex can match 2 consecutive dots (since all of the parts except for [^\s] are optional).

In order not to match strings like .4.5 or .. you just need to add the . to the [^\n] negated character class:

^([^\s.](-)?(\d+)?(\.)?(\d+)?)$
      ^

See demo. This will not let any . in the initial capturing group.

You can use a lookahead to only disallow the first character as a dot:

^(?!\.)([^\s](-)?(\d+)?(\.)?(\d+)?)$

See another demo

All explanation is available at the online regex testers:

enter image description here

In order to match the numbers in the format you expect, use:

^(?:[-]?\d+\.?\d*|-)$

Human-readable explanation:

  • ^ - start of string and then there are 2 alternatives...
  • [-]? - optional hyphen
  • \d+ - 1 or more digits
  • \.? - optional dot
  • \d* - 0 or more digits
  • | -OR-
  • - - a hyphen
  • $ - end of string

See demo

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9 Comments

What I trying to do is match any decimal number including negatives, but not empty string. 1 - Allowed -1 - Allowed 1. - Allowed -1. - Allowed 1.0 - Allowed -1.0 - Allowed
Sorry, '-' needs to be valid too. '-' Allowed '-.' not allowed '.' not allowed Please don't spent your whole time, you have already helped a lot
I'm doing a validation on keyup, user is writting the decimal number, he can starts with negative '-', but he can't start with dot '.', he can finish with dot, but he can't put more than one dot or more than one dash and not a dash before a dot.
Yeah, i think ^[-]?\d+\.?\d*|-$ will solve!! :) you're a good man.
@JohnnyWiller: Thanks for the reminder, I was focusing on other details and overlooked this. However, I'd rather use a non-capturing group since we are not interested in the captured text.
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