Should actually be:

onchange='checkconditions(this.value, this.name, this.type); var match = new RegExp(\"[,]\"); if (match.test(this.value)) {this.value = this.value.replace(\",\", \"\"); alert(\"Commas are not allowed, and have been removed. Please recheck the number and use a period if necessary.\");}'"

@c_schmitz: may you please have a look (=review) at the code provided by Pharmakon.

OnChange does not cut it here, since conditions and submission would take effect too late on survey submission.

Even if your user inserts characters by pasting the characters are sanitized to a number so there is no harm done.

To address point 1.
A javascript box is opened immediately when the user attempts to go to another input. Your explanation is inadequate.

Point 2:
It is NOT sanitized to a number properly. Here is a prime example;
A database is setup in the US - numbers in the US are formatted such as 3,000,000.00. Numbers in Europe are entered as 3.000.000,00. They are not the same(and yet are both considered valid numbers by LimeSurvey). Conversion from one standard to another must occur. It is best to catch this when it is being input. This is a valid concern for anyone who takes surveys on a global scale.

If you don't want to implement the method I posted - that's fine...no offense taken. I didn't tell you to, I informed you of the issue that needs to be resolved. Fix it another way - but this is a very serious data entry and potentially a security issue in this application.

1.) The javascript box is coming too late. A users can still manage to press a button to go to the next page.

2.) Before you start spitting around if this is 'potentially a security issue in this application' please get informed first how LimeSurvey works before spreading FUD - LimeSurvey is perfectly safe regarding that. The suggested method doesn't work properly and so does not solve the problem.

LimeSurvey does not yet support different number format. In fact, all numbers are treated like the x.xx decimal float format. Please file a feature request if you think that is not adequate - or better.. implement a number format as a question attribute and provide an according patch.
We will be happy to help if you have questions!