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

问题

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' (i.e., 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.

回答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. I.e., "'" 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.)

回答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.

来源：https://stackoverflow.com/questions/8586010/javascript-regexp-to-replace-a-value-only-when-it-follows-certain-characters

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javascript

regex