519 Despite my fascination, I am also happy to admit that regular

   520 expressions have their shortcomings. There are some well-known

   521 ``theoretical shortcomings'', for example recognising strings

   522 of the form $a^{n}b^{n}$. I am not so bothered by them. What I

   523 am bothered about is when regular expressions are in the way

   524 of practical programming. For example, it turns out that the

   525 regular expression for email addresses shown in \eqref{email}

   526 is hopelessly inadequate for recognising all of them (despite

   527 being touted as something every computer scientist should know

   528 about). The W3 Consortium (which standardises the Web)

   529 proposes to use the following, already more complicated

   530 regular expressions:

   531 

   532 {\small\begin{lstlisting}[language={},keywordstyle=\color{black},numbers=none]

   533 [a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*

   534 \end{lstlisting}}

   535 

   536 \noindent But they admit that by using this regular expression

   537 they wilfully violate the RFC 5322 standard which specifies

   538 the syntax of email addresses. With their proposed regular

   539 expression they are too strict in some cases and too lax in

   540 others. Not a good situation to be in. A regular expression

   541 that is claimed to be closer to the standard is shown in

   542 Figure~\ref{monster}. Whether this claim is true or not, I

   543 would not know---the only thing I can say it is a monstrosity. 

   544 However, this might actually be an argument against the 

   545 standard, rather than against regular expressions. Still it

   546 is good to know that some tasks in text processing just 

   547 cannot be achieved by using regular expressions.

   548 

   549 

   583 \begin{figure}[p]

   584 \tiny

   585 \begin{center}

   586 \begin{minipage}{0.8\textwidth}

   587 \lstinputlisting[language={},keywordstyle=\color{black},numbers=none]{../progs/email-rexp}

   588 \end{minipage}

   589 \end{center}

   590 

   591 \caption{Nothing that can be said\ldots\label{monster}}

   592 \end{figure}