201 Clearly, this string should clearly be identified as a keyword. The problem is that also the regular expression \textit{IDENT} for identifiers would also match this string. To solve this ambiguity we need to rank our regular expressions.

   201 Clearly, this string should clearly be identified as a keyword. The problem is that also the regular expression \textit{IDENT} for identifiers would also match this string. To overcome this ambiguity we need to rank our 

   202 

   202 regular expressions. In our running example we just use the ranking

   203 

   203 

   204 

   204 \[

   205 \textit{KEYWORD} < \textit{IDENT} < \textit{OP} < \ldots

   206 \]

   207 

   208 \noindent

   209 and so on. So even if both regular expressions match in the example above,

   210 we can give the regular expression for \ref{Page ??} as follows

   211 

   212 Let us see how our algorithm for lexing works in detail. The regular 

   213 expressions and their ranking are shown above.