Match an end html tag if start tag is not present

Question

i want to get an ending html tag like </EM> only if somewhere before it i.e. before any previous tags or text there is no starting <EM> tag my sample string is

ddd d<STRONG>dfdsdsd dsdsddd<EM>ss</EM>r and</EM>and strong</STRONG>

in this string the output should be </EM> and this also the second </EM> because it lacks the starting <EM>. i have tried

(?!=<EM>.*)</EM>

but it doesnt seem to work please help thnks

Answer 1

I am not sure regex is best suited for this kind of task, since tags can always be nested.

Anyhow, a C# regex like:

(?<!<EM>[^<]+)</EM>

would only bring the second </EM> tag

Note that:

• ?! is a negative look*ahead* which explains why both </EM> are found.
  So... (?!=<EM>.*)xxx actually means capture xxx if it is not followed by =<EM>.*. I am not sure you wanted to include an = in there
• ?<! is a negative look*behind*, more suited to what you wanted to do, but which would not work with java regex engine, since this look-behind regex does not have an obvious maximum length.

However, with a .Net regex engine, as tested on RETester, it does work.

Answer 2

You need a pushdown automaton here. Regular expressions aren't powerful enough to capture this concept, since they are equivalent to finite-state automata, so a regex solution is strictly speaking a no-go.

That said, .NET regular expressions do have a pushdown automaton behind them so they can theoretically cope with such cases. If you really feel you need to do this with regular expressions rather than a formal HTML parser, take a glimpse here.