Create a regex to replace the last occurrence of a character in a string

Question

I need to create a regex which should look for the last '*' irrespective of the whitespace in the string. Then, I need to replace that string with some text.

Currently, it is replacing the first occurence of '*' in the string.

How do I fix it?

Here's my code:

const regex = /\*/m;
const str = 'Field Name* * ';
const replaceStr = ' mandatory';
const result = str.replace(regex, replaceStr);
console.log('Substitution result: ', result);

Here, the output should be 'Field Name* mandatory'. But what I get is 'Field Name mandatory *'.

Answer 1

Instead of RegEx, use String#substring and String.lastIndexOf as below

const str = 'Field Name* * ';
const replaceStr = 'mandatory';
const lastIndex = str.lastIndexOf('*');
const result = str.substring(0, lastIndex) + replaceStr + str.substring(lastIndex + 1);

console.log('Substitution result: ', result);

Still want to use RegEx?

const regex = /\*([^*]*)$/;
const str = 'Field Name* * Hello World!';
const replaceStr = ' mandatory';
const result = str.replace(regex, (m, $1) => replaceStr + $1);
console.log('Substitution result: ', result);

Answer 2

Short regex magic (shown on extended input str):

const regex = /\*(?=[^*]*$)/m,
      str = 'Field Name* * * * ',
      replaceStr = ' mandatory',
      result = str.replace(regex, replaceStr);
console.log('Substitution result: ', result);

• (?=[^*]*$) - lookahead positive assertion, ensures that the former \* is matched only if it's followed by [^*]* (non-asterisk char right up to the end of the string $)