Linux Classroom Series – 11/Sept/2020

Regular Expressions

• They are also referred as regex, RegEx

• Truth about Regex:

  1. Most of the cases regex is defined as A regular expression is a declarative specification of describing the textual structure to match string
  2. The problem with above definition is regular expressions are not declarative, A regular expression is imperative. Regular expression is subroutine/function/method

• In What Language we write regular expressions. In Regular expressions we have six major dialects

  1. BRE:
     • This is basic regular expressions.
     • Tools: ed, sed, grep
  2. ERE:
     • GNU extended regular expressions
     • Tools & Languages : egrep, gawk, Notepad++, TCL
  3. EMACS:
     • This is Emacs regular expressions
     • Tools: Emacs
  4. VIM:
     • TOOLS: VIM.
  5. PCRE:
     • This is PERL(5) compatible regular expressions
     • TOOLs & Languages: PERL, .NET, APACHE, C#, Java, JavaScript, PHP, Powershell, Python, R, Ruby,
  6. PSIX:
     • Perl 6 Regular Expressions
     • Languages: Perl 6

• These dialects have similarities & dissimilarities. There are relationships b/w dialects

• How are regular expressions implemented?

  • Theoretically Regular expression are implemented on a Finite State machine(FSM). But Languages Practically implement regular expressions on stack-based machine.
  • To understand regular expressions we will be using FSM
  • To search for a word cat in the text sequence in all six dialects mentioned above the regex is /cat/
  • Lets represent cat in transition graph
  • If the above regular expression is represented by code

  for(index=0; index<len(message); index++) {
      match_position = index
  
      try {
  
           message[match_position] == 'c' or throw Backtracking
           match_position++;
  
           message[match_position] == 'a' or throw Backtracking
           match_position++;
  
           message[match_position] == 't' or throw Backtracking
           match_position++;
  
          return TRUE;
  
  
  
      }
  }
  return FALSE;