Regex for disallowing commas

Question

I'm trying to disallow commas in a string entered into a textbox. Here is what I have so far:

[RegularExpression (@"?[^,]*$",
        ErrorMessage = "Commas are not allowed in the subtask title. Please remove any and try again")]

This is probably my 5th or 6th attempt, nothing so far has worked. Any help would be appreciated.

Solution

Try changing your regex to:

"^[^,]+$"

Let's say we're matching against "Hello, world"

The first ^ asserts that we're at the beginning of the string. Next [^,] is a character class which means "Any character except ,." + next to something means "Match this one or more times." Finally, $ asserts that we're now at the end of the string.

So, this regular expression means "At the start of the string (^), match any character that's not a comma ([^,]) one or more times (+) until we reach the end of the string ($).

This regular expression will fail on "Hello, world" - everything will be fine for H, e, l, l, and o, until we reach the comma - at which point the character class fails to match "not a comma".

For some great tutorials about regular expressions, you should read up on http://www.regular-expressions.info/

Comments

[Daniel]

So what does the * do? In @avinashRaj response, he used the * character which did exactly what I wanted. What's the difference between that and +?

[Rickkwa]

@Daniel * is 0 or more, + means at least 1

[Kelvin]

* means 0 or more times - or in other words, "this is optional". For example, if you wanted to match "http" and "https" and didn't care which it is as long as both match, you would use "https?" ("match h, match t, match t, match p, and match s if it's there but don't worry if it's not")

[Daniel]

So why does the * work at all? If it is 0 or more, shouldn't it be considering the whole string invalid no matter what the input is?

[Daniel]

So it seems that the + is actually the correct way to accomplish this then? The * could have some unintended consequences?

[Kelvin]

The * works because [^,]* means "Match anything that's not a comma 0 or more times" - so, because we have ^ and $ matching the start and end of the string, we need everything between the start and end of the string to be anything that's not a comma. The only literal difference, in this case, is that * will allow an empty string, whereas + demands that it's at least one character long.