How to dynamically create regular expressions based of a user input

Question

I am making a program that gets an input from the user, then makes a regular expression using that input.

Here is some example code:

<input id="a" type="text" />
<input type="button" value="Create" id="b" />
<script>
(function (z , code) {
    var input = document.getElementById("a"),
        b = document.getElementById("b");
    function primary(){
        var regEx = new RegExp("[A-Z]{3}[ ]\d{9}[ ]" + input.value.toUpperCase(), "g");
        alert(regEx);
    };
    b.addEventListener("click",function(){
        primary();
    }, false);
}(this, document));
</script>

FIDDLE

It works accept for this:

Say I use FooBar as my input. Instead of creating a regular expression that says, /[A-Z]{3}[ ]\d{9}[ ]FOOBAR/g I get a regular expression that says, /[A-Z]{3}[ ]d{9}[ ]FOOBAR/g. A d instead of \d.

Why is this not working properly? How can I correct this?

Thank you.

When you use the regex constructor you need to double escape special charaters, so \\d, etc. — elclanrs
– elclanrs, Commented Jan 23, 2014 at 3:45
@elclanrs Thanks! That sure fixed it! If you posted that as an answer I could mark it as correct. — Progo
– Progo, Commented Jan 23, 2014 at 3:47

thefourtheye · Accepted Answer · 2014-01-23 03:52:50Z

2

As per the MDN RegExp docs,

When using the constructor function, the normal string escape rules (preceding special characters with \ when included in a string) are necessary. For example, the following are equivalent:
var re = /\w+/;
var re = new RegExp("\\w+");

In your RegEx, \d is a special character

\d Matches a digit character in the basic Latin alphabet. Equivalent to [0-9].

For example, /\d/ or /[0-9]/ matches '2' in "B2 is the suite number."

So, your RegEx, should have been like this

[A-Z]{3}[ ]\\d{9}[ ]   //\\d instead of \d

answered Jan 23, 2014 at 3:52

thefourtheye

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