Question

 var rxDatePattern = new RegExp("^(?:\\d*\\.\\d{" + no + "})$");

This is the regular exp pattern im using to accept numeric values with 3 digits after decimal.The value is stored in the variable no….
This is working fine , but i kind of did not understand the pattern , can anybody help me the pattern..
Thanks

Answer 1

^ and $ are anchors for the start and end of string, respectively. They make the regex match a complete string in this case instead of just a substring.

\d stands for a single digit, while \d* is, due to the * quantifier, a series of at least zero digits. Then follows a literal dot, matching, well, a dot (.) and a variable number of digits again, e.g. \d{3}.

The reason why the regex contains \\d instead of \d is that \ is also the escape character for strings in JavaScript. So to include a literal backslash in a regex that is contained in a string you have to escape the backslash once, so the regex engine sees an actual backslash.

Finally all this is enclosed in a non-capturing group (?:...), although I don’t quite see why. It has no effect in this case and could be left out as well.

So for no = 2 this would match strings like

1.23
123.45
.00

but not strings like

.123
1.123
5.3
2
1.