Whats the difference between sed -E and sed -e

Question

I\'m working on some old code and I found that I used to use

sed -E \'s/findText/replaceWith/g\' #findText would contain a regex

but I no

Answer 1

From source code, -E is an undocumented option for compatibility with BSD sed.

/* Undocumented, for compatibility with BSD sed.  */
case 'E':
case 'r':
  if (extended_regexp_flags)
    usage(4);
  extended_regexp_flags = REG_EXTENDED;
  break;

And from manual, -E in BSD sed is used to support extended regular expressions.