For my parser, for the purpose of this question, any line starting with a single lowercase letter among a set of lowercase letters, followed by the character '=' followed by any other character is a valid line. So, the following are valid lines (all starting from first column):
a=20
b=50 70
q=20 Hello There
z=-
Any other line is not valid. My need is to match the complement. How do I write a flex expression to match the invalid lines. My confusion arises from the ^ which means start of line as well as complement the expression. I thought ^[abq][=].+ would match the acceptable line so merely complementing it with ^ will do. But ^ at the start of the expression always implies match at start of the line. I made a few other attempts but that did not work too. Though not relevant, the expression is used as the first step to discard invalid SDP lines. See here for details from the relevant SDP RFC, if it matters.
The simplest approach is to always match entire lines (or use different start conditions to lexically analyse the rest of valid lines). Although flex does not have a negation operator (the [^…] negative character class is not an operator), in this case the expressions are pretty simple and can be expressed easily enough. Note that it doesn't matter that the various "invalid line" patterns are not disjoint, since it doesn't matter which one matches a particular invalid line. So here are three patterns which I believe collectively match all invalid lines
[^abqz\n].* { /* Starts with the wrong letter */ }
.[^=\n] { /* Second character not = */ }
.$ { /* Only one character in line */ }