I am a monk in training, and I have a regular expression that matches on any text that would be considered a token by a C++ compiler for the purpose of counting lines of code vs comments. Matching against the regex itself is fine; matching against the regex "+" breaks. The conditions for a token string for my purposes are:

EITHER:

• at least one character long
• no whitespace
• no single or double quote marks (keeping in mind that the trigraph ??' is actually a ^)
• no C or C++ comments (keeping in mind that the trigraph ??/ is actually a \)

OR:

• begins and ends with either ' or " (and ending with the same one it begins with)
• escaped quote marks are allowed inside the quoted string, i.e. \', \", ??/', and ??/" are OK
• trigraph ??' does not close a single-quoted string

Having somewhat of a background in computing theory, I used state machines to derive "pure" regular expressions (i.e. no lookahead, stateful matching, or anything like that) and ended up with this for a token:

(((\?+|(\?\/|\/)(\?\/)*\?*)|([^\'\"\/\s\?]|((\?|\/)\?+[^\s\?\"])|((\?\
+/|\/)(\?\/)*([^\'\"\/\s\?\*]|\?(\?+[^\?\"\s]|[^\'\"\/\s\?]))))([^\'\"
+\/\s\?]|\?\?+[^\?\"\s]|(\/|\?\/)(\?\/)*([^\'\"\/\s\?\*]|\?(\?+[^\?\"\
+s]|[^\'\"\/\s\?])))*((\?+|(\/|\?\/)(\?\/)*\?*)?))|((\'([^\'?\\]|\\.|\
+?(\?+(\/.|[^?\/])))*(\'|\?\'))|(\"([^\"\\?]|\\.|\?\?+(\/.|[^\?\"\/]))
+*(\"|\?+\"))))
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while( $line = <> )
{
  if( $line =~ m/^$token$/ )
  {
    print "match\n";
  }
  else
  {
    print "mismatch\n";
  }
}
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x
match
/*x*/
mismatch
"x"
match
"\"
mismatch
"\""
match
"/*x*/"
match
"x"/*x*/
mismatch
/*x*/x
mismatch
x/*x*/
mismatch
//x
mismatch
x="2";
mismatch
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(blank)
mismatch
x
match
x="2";
match
/*x*/
match
//x
match
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