Beginners guide for Regular Expressions(regexp)

Top 10 basic regexp to know for Ansible beginners while using "replace" or "lineinfile" modules

1. Start with Special character.

(dot) .

It selects all the content, expect new lines. example

2. To avoid and just want to select the literal character of (Dot). then use literal MetaCharacter as below

(back slash) \

3. When we want to select the exact word, as it is case senstive.

ex:  try with "case sentive" vs "Case Sensitive" 

4.  When there is a repetition of the same word, but we need to select the word or character that begins the line.

     Beginning of a string - "^" 

 ex:  ^xploritec.com

   

5.  When we want to filter more specific word that starts with and end with.

     Use Anchors "^" - Beginning of String and "$" - End of String

     The below example has xploritec.com in two places but we need a specific one that starts at beginning of a line.

ex: ^xploritec.com$

6.  Select a specific word that has a boundary.

      ex: "GODOFFIRE Prometheus"  and "GODOFFIREPrometheus" 

when we just filter the word Prometheus, it will select all the matching words with or without boundaries. using "\bPrometheus" filter matches that have a Boundary as shown below. 

 "\b"

  \bPrometheus

     

7. Opposite to the above selection, the word that should not have a Boundary.

  "\B"

   \BPrometheus

8.  At times we want to filter numbers that matches certain pattern.

     ex: Let's assume we have a 10 digit phone number, but it has no standard format and now we want to filter all the match cases. 

"\d" "{}-quantifier" "dot(.) - selects any"  

\d{3}.\d{3}.\d{4}

9. when we want to filter the exact literal match of a standard format along with specific starting area codes. 

ex: 55* 66* 56* 65* - we can write as [56][65] as starting two numbers.

we can use character set [ ]

^[56][56]0-\d{3}-\d{4}

10.  At times we have to filter and replace a combo of a few characters or words which has both numbers, special characters, and letters(including upper and lower cases)

ex: replace all the websites, emails, servernames etc with company proxy server.

we can use groups"()" and quantifiers"[]" to acheive this.

[a-zA-Z0-9-.]+(@|.)[a-zA-Z]+\.(com|edu|org)

Exact realtime Ansible examples will be coming in next slide.