reorganisation par cours dans sous-répertoires

2-InformationTheory/2InformationTheory.md

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+Cours de Florent.
+
+# Information Theory
+
+## A quick introduction to regular expressions.
+
+Regular expressions (regexp for short) provide an effective tool to define languages.
+The correspondance with finite automata mean that it is possible to efficiently compile a regular expression into an automaton that recognises the corresponding language.
+
+On Linux this is precisely what the command grep does.
+
+We will explain how an automaton can be generated from a regexp and see how to use the grep command to solve riddles.
+
+### Ingredients of classical regexp.
+
+```
+the letters of the alphabet
+
++ means or
+used as L1 + L2 where L1 and L2 are two languages
+means a word of L1 or a word of L2.
+denotes the union of the languages
+
+. means concatenation 
+used as L1.L2 
+means a word of L1 followed by a word of L2
+
+* means repetition 0 or more times
+used as L*
+means the empty word epsilon (0 repetition) or one or more words of L one after another
+equivalent to epsilon + L + L.L + L.L.L + L.L.L.L + ...
+```
+
+### Construction : from regexp to automaton
+
+We allow for automaton that allow transitions labelled with epsilon.
+Then we show how to do without them.
+
+Details on the board.
+
+Note that JFLAP proposes an activity for this construction.
+
+There is also an inverse transformation from automaton to regexp, also available on JFLAP.
+This shows that languages defined by a regexp and languages recognized by a finite automaton form the same class of languages, commonly known as regular languages.
+
+
+### grep
+
+We shall in fact use the extended regular expressions of grep.
+Use the command egrep or grep -e.
+
+See the manual of grep for the syntax.
+
+### Some additionnal commands
+
+* tr to replace a character by another
+* grep to search for some regular expression line by line
+* wc to count words (or characters)
+* sort to sort the lines of a file
+
+### Exercise
+
+Wordle.
+
+Demo.