JavaScript Regexp to replace a value only when it follows certain characters?

Question

I'm trying to come up with a regular expression which will search for a string and replace with new set of characters. My string is something like this.

(Product=='Partnership - CT'&&State==CT)

Here I have to search for the CT value for state key and replace with 'CT' (ie, add single quotation marks). I have used the following Regexp in JavaScript.

var expressionQuery = "(Product=='Partnership - CT'&&State==CT)";
val = 'CT';
expressionQuery.replace(new RegExp(val, "g"),"\'"+val+"\'" );

But this is giving output as (Product=='Partnership - 'CT''&&State=='CT')

Expected output : (Product=='Partnership - CT'&&State=='CT')

That is, I only want to change the instance of CT that appears immediately after ==. Please help me to write the expression to fix this.

Answer 1

UPDATE: You don't have to hardcode the "CT" in your regular expression. If you know that the "State" parameter will always be a two-letter code you can add quotation marks to any state with something like the following:

expressionQuery.replace(/State==([A-Z]{2})/g,"State=='$1'");

This says to find a the substring "State==" followed by any two letters, and then in the replace expression where it has "$1" that means insert whatever matched the part of the regex in parentheses. Note that because it specifically looks for a letter after "==" it won't add extra quotation marks if they were already present.

And my original answer, which replaces only the specified state (whether hardcoded or otherwise indicated by a variable):

You could go for non-capturing matches on the "==" and so forth, but I think it would be easier to just include "==" in both the search and replace text:

var expressionQuery = "(Product=='Partnership - CT'&&State==CT)",
    val = "CT";

expressionQuery.replace(new RegExp("==" + val, "g"),"=='"+val+"'" );

Or you may even want to include "State" in the search, in case you later have other parameters that might match, like "SomeOtherParam==CT":

expressionQuery.replace(new RegExp("State==" + val, "g"),"State=='"+val+"'" );

Note that you don't need to escape single quotation marks if they are in a string quoted with doubles. Ie, "'" does the same thing as "\\'" .

(Note also that your original code was invalid because you forgot to put any quotation marks around the string in your declaration of expressionQuery .)

Answer 2

You could use a negative lookahead to ensure that there is no ' ahead of the pattern you want to replace.

expressionQuery.replace(new RegExp('CT(?!\')', "g"),"\'"+val+"\'" );

It works for your example, but I don't know if this rule is strict enough to avoid false replacements in all cases.