How to Use Awk and Regular Expressions to Filter Text or String in Files

When we run certain commands in Unix/Linux to read or edit text from a string or file, we most times try to filter output to a given section of interest. This is where using regular expressions comes in handy.

Read Also: 10 Useful Linux Chaining Operators with Practical Examples

What are Regular Expressions?

A regular expression can be defined as a strings that represent several sequence of characters. One of the most important things about regular expressions is that they allow you to filter the output of a command or file, edit a section of a text or configuration file and so on.

Features of Regular Expression

Regular expressions are made of:

1. Ordinary characters such as space, underscore(_), A-Z, a-z, 0-9.
2. Meta characters that are expanded to ordinary characters, they include:
   1. (.) it matches any single character except a newline.
   2. (*) it matches zero or more existences of the immediate character preceding it.
   3. [ character(s) ] it matches any one of the characters specified in character(s), one can also use a hyphen (-) to mean a range of characters such as [a-f], [1-5], and so on.
   4. ^ it matches the beginning of a line in a file.
   5. $ matches the end of line in a file.
   6. \ it is an escape character.

In order to filter text, one has to use a text filtering tool such as awk. You can think of awk as a programming language of its own. But for the scope of this guide to using awk, we shall cover it as a simple command line filtering tool.

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