CODEWITHSUNDEEP
javascriptregex
Awani
Awani

Reputation: 394

Usage of \. in regex in javascript

I have the following regular expression which performs very basic validation to allow only integers and should not allow any floating point numbers and negative numbers. I found the below expression as one of the ways to do so:

var reg = new RegExp('^(([0-9]+)|\.[0-9]+)$');

This expression correctly validates the following inputs:

validatePercentage(56.67); --> not an integer
validatePercentage(67); --> integer
validatePercentage(-5667); --> should not be negative

However, I'm unable to understand the usage of '\.' in the expression (that somehow seems to be making a difference to the output). Can somebody please explain how exactly is the regex working to eliminate negative inputs? Thanks in advance.

Upvotes: 1

Views: 93

Answers (2)

Quentin
Quentin

Reputation: 943240

  1. . in a regex means "any character"
  2. \. in a regex means "a period character"
  3. \. in a JavaScript string literal means ..

This code is an attempt at using 2, but because the regex is being constructed using a string (almost always a mistake) it ends up being a 3 which gets converted into a 1.

The regex appears to be an attempt at "Zero or a positive integer or a positive float that is less than one" but is mangled by the string phase to be "Zero or a positive integer either of which may be preceded by any character at all".

The correct regex would probably be something more like:

var reg = /^[1-9][0-9]+(?:\.0+)?$/

If you really want a test for a positive integer then you'd probably be better off with:

value == parseInt(value, 10) && value > 0

(or > -1 if you want to allow 0 as a value).

Upvotes: 9

Casimir et Hippolyte
Casimir et Hippolyte

Reputation: 89557

To figure a literal dot in a pattern, the dot must be escaped, otherwise the dot may match any characters except newlines.

When you use the object notation with a javascript string to define a pattern, backslashes must be escaped that gives:

var reg = new RegExp('^(([0-9]+)|\\.[0-9]+)$');

or

var reg = /^(([0-9]+)|\.[0-9]+)$/;

You can write too:

var reg = new RegExp(/^(([0-9]+)|\.[0-9]+)$/);

Upvotes: 3

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