Advanced Regular Expression Features

If you want to employ all regular expressions constructs and features you can't make it with a simple shell command or a tool, but you need to use a programming language. I use Perl, because it has the best and longest established support for regular expression.

Parenthesized Subexpressions

Earlier, we saw that regex engines support metacharacters \1, \2, \3, etc. to refer to the text matched by parenthesized subexpressions. Perl and most other modern regex-endowed languages also provide a way to refer to the text matched by parenthesized subexpressions from code outside of the regular expression, after a match has been successfully completed.

Introduction to Perl

This guide introduces the Perl language through a number of sample programs. The aim of this guide is to understand how to use regular expressions with Perl.

1 - What is Perl Language

Perl is a scripting language suitable for web-processing. Perl was developed deriving its concepts of text handling and regular expression from two specialized languages called awk and sed.

Regular expressions Introduction

1. What is a regular expression?

"A regular expression is a pattern which specifies a set of strings of characters; it is said to match certain strings."[Ken Thompson]

From the English dictionary:

pattern = (noun) a regularly repeated arrangement of something.
to match = (verb) to be equal.

A regex can or cannot match a string and, if it does, can match the string once or multiple times.

The <script> element

The unobtrusive javaScript programming paradigm

Unobtrusive JavaScript is an approach to web programming that supports separation of behaviour (JavaScript code), structure (HTML code) and presentation (CSS code).
According to this programming paradigm, to accomplish separation is best to write all your JavaScript in an external file and embed it in HTML document using a <script> element with src attribute.

The <script> element is synchronous by default

Let's examin how the <script> element allow you to insert javascript in a HTML document.

<script src="scripts/javascript_app.js"> </script> <!--synchronous script-->

When JavaScript engines encounters a <script> element, they stop parsing HTML to download and interpreter the JS file, then they resume parsing and rendering HTML.

The <script> element has two important attribute: HTML 4.1 defines the async attribute and HTML5 introduces the defer attribute. These two attributes cause scripts to be executed differently, in browsers supporting them.

JavaScript Execution Contexts

1 - Execution Context or Context

The execution context or context is the environment where JavaScript code is evaluated.

→ Global (execution) context

The global execution context is the most outer execution context.

In browsers the global context is also known as window context. In a browser global variables are window properties and global functions are window methods. 

→ Function or Local (execution) context

When a function is called a function execution context is created.