115 

   115 

   116 \defn{Regular expressions} help with conveniently specifying

   116 \defn{Regular expressions} help with conveniently specifying

   117 such patterns. The idea behind regular expressions is that

   117 such patterns. The idea behind regular expressions is that

   118 they are a simple method for describing languages (or sets of

   118 they are a simple method for describing languages (or sets of

   119 strings)\ldots at least languages we are interested in in

   119 strings)\ldots at least languages we are interested in in

   121 expression for describing the English language short of

   121 expression for describing the English language short of

   122 enumerating all English words. But they seem useful for

   122 enumerating all English words. But they seem useful for

   123 describing for example simple email addresses.\footnote{See

   123 describing for example simple email addresses.\footnote{See

   124 ``8 Regular Expressions You Should Know''

   124 ``8 Regular Expressions You Should Know''

   125 \url{http://goo.gl/5LoVX7}} Consider the following regular

   125 \url{http://goo.gl/5LoVX7}} Consider the following regular

   213 

   213 

   214 \subsection*{Why Study Regular Expressions?}

   214 \subsection*{Why Study Regular Expressions?}

   215 

   215 

   216 Regular expressions were introduced by Kleene in the 1950ies and they

   216 Regular expressions were introduced by Kleene in the 1950ies and they

   217 have been object of intense study since then. They are nowadays pretty

   217 have been object of intense study since then. They are nowadays pretty

   219 implementing regular expressions. I am sure you have come across them

   219 implementing regular expressions. I am sure you have come across them

   220 before (remember the PRA or PEP modules?). 

   220 before (remember the PRA or PEP modules?). 

   221 

   221 

   222 Why on earth then is there any interest in studying them again in depth

   222 Why on earth then is there any interest in studying them again in depth

   223 in this module? Well, one answer is in the following two graphs about

   223 in this module? Well, one answer is in the following two graphs about