title: Regular expression of a string that starts with a letter followed by only letters and digits except two specific strings in lex
body: <p>I am trying to write the regular expression of a string  that has the characteristics mentioned above. Here, the specific strings are "true" and "false". For example:"d6" will be accepted but "6d" won't. Also "true" and "false" won't be accepted. I have googled a lot, got various examples but still can't make it. Please help.</p>