# Regular Expressions

First of all, this will be a bit painful but as with `vim` once you overcome the initial learning curve you start to see the potential regular expressions bring to the table.
To make matters even worse, there are multiple *flavors* of regexes.
An overview and comparison between different flavors can be found on [wikipedia](https://en.wikipedia.org/wiki/Comparison_of_regular-expression_engines).
Don't see this as a reason *not* to learn some basic expressions though, a little experience goes a long way.

## What are they?

> A regular expression (shortened as regex or regexp;[1] also referred to as rational expression[2][3]) is a sequence of characters that specifies a search pattern. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation. It is a technique developed in theoretical computer science and formal language theory.

[wikipedia](https://en.wikipedia.org/wiki/Regular_expression)

You can see regular expressions as find (and replace) on steroids.
As a practical example, I used *a lot* of regular expressions to clean up the multiple choice LPI questionnaires.
This was done in `vim` so I used the vim flavor regex but it's not too much different from the main one you should know, `grep`.

From a practical system administer point of view you'll probably use regexes in this order:

1. with `grep`
2. with `sed` (went copy pasting commands found online)
3. with `vim`
4. with a scripting language such as `python3`

## How to learn them?

Some tips and pointers before we head into the actual syntax.

### Vim

There is a setting in `vim` that is disabled by default but highly advised to learn vim regexes.
By setting `set incsearch` in your `~/.vimrc` or in the **expert** command line vim will highlight whatever matches the pattern you're searching for.
This can be a tremendous help when building complex patterns.

### Grep

By default `grep` only interprets basic regular expressions.
If you want, or more likely *need* to use [extended](https://www.gnu.org/software/grep/manual/html_node/Basic-vs-Extended.html) expressions you should use `grep -E` or `egrep` instead.
For completeness's sake I should mention there is a third *version* of `grep` invoke with `grep -P` that interprets the patterns as [perl regex](https://perldoc.perl.org/perlre).
One of the advantages of perl regexes is reverse matching.

## The basics



## Exercises

Below are some practical exercises and files to go with them.
Use them to test out you grepping skills and as inspiration for personal challenges.

* configuration [file](./assets/sysctl.conf)
	* print only lines with actual configuration settings (ignore comments)
* css [file](./assets/teddit.css)
	* extract all the hex color codes
* html [file](./assets/teddit.html)
	* html extract pictures
		* just jpg
		* jpg and png at the same time
* log [file](./assets/auth.log)
	* extract all IP addresses
		* plus only the unique ones
	* extract all wrong logins for known users
	* extract all unknown users (this is tricky and requires backwards searching using `grep -P`)
* mail dump [file](./assets/dump.mail)
	* extract all unique email addresses
	* extract all web links
		* only the base link (https://www.example.co.uk)
		* both http and https links

There are some very good regex exercises online as well.
[This](http://regextutorials.com/) is a good starting point.