Obviously, this is written by someone who thinks all strings must be doube-quoted in Perl! And consequentely he put double quote around the regex. Not only this is not necessary, it actually breaks the regex.

Most probably, it was meant to be:

if (/^abc[A\b]def$/ ){
    print "true"    ;
}
else
{
    print "false";
}
[download]

Which means: match a string starting with 'abc', followed by either a capital 'A' or a backspace, followed by 'def' which ends the string.

Please note that normally \b in a regex means "match on a word boundary", but inside a character class (the square brackets) it means 'backspace'.

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