Use /b[A-Z]{2,}b/g to match all-caps words and then .replace() with a callback that lowercases matches.

var string = It's around August AND THEN I get an email,

  regex = /b[A-Z]{2,}b/g;



var modified = string.replace(regex, function(match) {

  return match.toLowerCase();

});



console.log(modified);

// It's around August and then I get an email

Also, feel free to use a more complicated expression. This one will look for capitalized words with 1+ length with I as the exception (I also made one that looked at the first word of a sentence different, but that was more complicated and requires updated logic in the callback function since you still want the first letter capitalized):

b(?!Ib)[A-Z]+b