Pass a variable into regex with string replace

Question

I have a complicated regEx inside a string replace and I want to use a variable called "tagname" instead of the word Category, but I don't know how to do this.

$("textarea").val().replace(/\<Category\>(.*)\<\/Category\>/gi, "<Category>TextVariable</Category>");

I have tried something like this, but it didn't work in the first parameter.

$("textarea").val().replace("/\<" + tagname + "\>(.*)\<\/" + tagname "\>/gi", "<" + tagname + ">" + TextVariable + "</" + tagname + ">");

Answer 1

You can use the RegExp constructor with a string pattern and flags:

var regex = new RegExp("<" + tagname + ">(.*)</" + tagname + ">", "gi");
$("textarea").val().replace(regex, "<" + tagname + ">" + TextVariable + "</" + tagname + ">");

Answer 2

Use the RegExp constructor instead of regex literal.

var regex = new RegExp("<" + tagname + ">(.*)</" + tagname + ">", "gi");
$("textarea").val().replace(regex, "<" + tagname + ">" + 
   textVariable + "</" + tagname + ">");

you can simplify it as:

//capture the tag name:
var regex = new RegExp("<(" + tagname + ")>(.*)</" + tagname + ">", "gi");
$("textarea").val().replace(regex, "<$1>" + textVariable + "</$1>");

But bear in mind that parsing xml/html with regex is a baaad idea