Negative lookahead

Negative lookahead can be used to see if the next element would match a certain pattern. It's often used to prevent a greedy construct from matching a special sequence. For example when matching a quoted string everything should be ignored except for the closing double quote. Using negative lookahead we could do something like this:

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"\"" {~"\"" .} "\""

Which means:

• "\"" = match a double quote
• {~"\"" .} the braces mean repeat the contents:
  • {~"\"" = if it is not a double quote do the next element. In other words, if you meet a double quote, then end the repeat.
  • . (the full stop) = ignore anything except a new line
• "\"" = match another double quote, which must be the end of the quoted text. This is the double quote which ended the Repeat process.

Ignored Character Set

In this case we could also have used an ignored character set instead of a negative lookahead, i.e. replace {~ "\" " .} with {'~"'} So we get

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"\"" {'~"'} "\""

The ignored character set { ' ~ " '} wil match anything other than a double quote. (I have added spaces to that expression to reduce confusion as Netsurf won't do character-spacing)

You may like to try this on the sample text - you should find four such quoted strings.