I wanna check the string and if there is a double quotation first check if the previous character is not backslash, then replace it with \".
I used below approach but I didn't know how to check previous character by regex
str = 'field_id + "content" ';
result = str.replace(/\"/g, '\\"');
// result : 'field_id + \"content\"'
str_2 = 'field_id + \"content\" ';
result_2 = str_2.replace(/\"/g, '\\"');
// result_2 : 'field_id + \\"content\\" '
// desired result => 'field_id + \"content\" '
One approach would be to do a regex replacement on all quoted terms, and then build the output using backslash escaped double quotes.
var str = 'field_id + "content" ';
var output = str.replace(/"(.*?)"/g, '\\"$1\\"');
console.log(output);In your regular expression you escape the " (\"). That's not necessary.
Without it, you get the desired result:
const doubleApoStrophRE = /"/g;
let str = 'field_id + "content" ';
let str2 = 'field_id + \"content\" ';
console.log(str.replace(doubleApoStrophRE, '\\"'));
console.log(str2.replace(doubleApoStrophRE, '\\"'));
// for str3 you need a bit more complexity
let str3 = `field_id + \\\\\\"content\\\\\\"'`;
console.log(str3.replace(/(\\{2,})?"/g, '\\"'));Check regex101 for trying out regular expressions