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Learning AWK Programming
Learning AWK Programming

Learning AWK Programming: A fast, and simple cutting-edge utility for text-processing on the Unix-like environment

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Learning AWK Programming

Working with Regular Expressions

AWK is a pattern-matching language. It searches for a pattern in a file and, upon finding the corresponding match, it performs the file's action on the input line. This pattern could consist of fixed strings or a pattern of text. This variable content or pattern is generally searched for with the help of regular expressions. Hence, regular expressions form a very important part of AWK programming language. In this chapter, we will look at regular expressions and how they are handled with AWK.

In this chapter, we will cover the following:

  • Regular expressions and their usage in AWK
  • The basic regular expression construct
  • Understanding the metacharacters of regular expression
  • The precedence of regular expressions
  • GAWK-specific regular expressions
  • Case-sensitive matching
  • Escape sequences

Introduction to regular expressions

In this section, you will learn about regular expressions and why we use them. Then we will discuss the usage of regular expressions in AWK.

What is a regular expression?

A regular expression, or regexpr, is a set of characters used to describe a pattern. A regular expression is generally used to match lines in a file that contain a particular pattern. Many Unix utilities operate on plain text files line by line, such as grep, sed, and awk. Regular expressions search for a pattern on a single line in a file.

A regular expression doesn't search for a pattern that begins on one line and ends on another. Other programming languages may support this, notably Perl.

...

Basic regular expression construct

Regular expressions are made up of two types of characters: normal text characters, called literals, and special characters, such as the asterisk (*, +, ?, .), called metacharacters. There are times when you want to match a metacharacter as a literal character. In such cases, we prefix that metacharacter with a backslash (\), which is called an escape sequence.

The basic regular expression construct can be summarized as follows:

Here is the list of metacharacters, also known as special characters, that are used in building regular expressions:

\ ^ $ . [ ] | ( ) * + ?

The following table lists the remaining elements that are used in building a basic regular expression, apart from the metacharacters mentioned before:

Literal

A literal character (non-metacharacter ), such as A, that matches itself.

Escape...

Understanding regular expression metacharacters

When we use special characters in regular expressions, they are called metacharacters because they have a special meaning. They enhance the flexibility, power, and usage of regular expressions. Now, we will discuss the list of metacharacters in regular expressions. All characters, that are not either metacharacters or escape sequences, match themselves in regular expressions. Let's understand each of them in detail.

Quoted metacharacter

A regular expression consisting of a literal or character (letter or digit) is a basic regular expression that matches itself. Sometimes, if we need to use the literal meaning of a metacharacter in a regular expression, we precede it with...

Precedence in regular expressions

In regular expressions, the '|' alternation operator has the lowest precedence, then concatenation, followed by the +, *, and ? repetition operations as well as brackets, '{' and '}'. As in arithmetic expressions, operators of higher precedence are done before lower ones. Also, parentheses can change how operators are grouped. Due to these conventions, parentheses are often omitted, as follows:

$ awk '/Jane|Emily/' emp.dat is same as
$ awk '/(Jane)|(Emily)/' emp.dat

The output on execution of the preceding code is as follows:

Jane    Kaur    9837432312  jane@gmail.com      F   hr      1800
Emily Kaur 8826175812 emily@gmail.com F Ops 2100

In POSIX, AWK, and GAWK, the '*', '+', and '?' operators stand for themselves when there is nothing in the regular...

GAWK-specific regular expression operators

GAWK has some more regular expression operators, in addition to those we've already discussed. The operators described in this section are specific to GAWK and are not available in other AWK implementations. These are basically shorthand character classes that are frequently used, and most of them deal with word matching (here, word refers to a sequence of one or more letters, digits, or underscores). We access them by using the escape character, '\', followed by a letter (here, the escape character adds special meaning to them instead of taking it away).

Let's create a file to practice GAWK-only operators:

vi  gopr.txt
start
bigstarting
starting
start end
white space example
e d
end
e_d
e1d
e;d
eNd
white space example with tab
e d
...

Case-sensitive matching

In bracket expressions, we can use both lowercase and uppercase characters to match the characters for case-insensitive results, but the same is not true for other regular expressions. However, this becomes difficult if you have to match to many characters. Here comes the two built-in string functions of AWK, tolower() and toupper(). Using these two functions, we can perform case-insensitive match operations.

For example, to match the first name 'John' of an employee in the emp.dat file, we can perform a case-insensitive match using the tolower() function, as follows:

$ awk 'tolower($1) ~ /john/' emp.dat

The output on execution of this code is as follows:

John    Kapur   9911556789  john@gmail.com      M   hr      2200

Another way to perform a case-insensitive match operation is by setting the IGNORECASE bash variable value to TRUE.

...

Escape sequences

When using regular expressions and strings, certain characters cannot be included literally in string constants ("hello") or regular expression constants (/hello/). To specify the characters for which there may not be other notations, they are represented as escape sequences. Escape sequences begin with a backslash ('\'). For example, it can represent that an \n newline character is used, which otherwise cannot be represented in strings or regular expressions. Similarly, \b stands for backspace, \t stands for tab, and \/ represents a forward slash. They are quite useful in generating formatted output.

For example, to insert a new line after each line of the emp.dat file, we can use the \n escape sequence, as follows:

$ awk '{ print $0"\n" }' emp.dat

The output on execution of the given code is as follows:

Jack    Singh ...

Summary

In this chapter, we learned about regular expressions. We learned the regular expression construct using literals and metacharacters. Then, we learned the usage of various different regular expression metacharacters, which are also known as regular expression operators. Finally, we covered some GAWK-specific regular expression metacharacter usage and understood escape sequences and their role in regular expressions.

In the next chapter, we will learn about handling AWK variables and constants while writing AWK programs.

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Key benefits

  • -Master the fastest and most elegant big data munging language
  • -Implement text processing and pattern matching using the advanced features of AWK and GAWK
  • -Implement debugging and inter-process communication using GAWK

Description

AWK is one of the most primitive and powerful utilities which exists in all Unix and Unix-like distributions. It is used as a command-line utility when performing a basic text-processing operation, and as programming language when dealing with complex text-processing and mining tasks. With this book, you will have the required expertise to practice advanced AWK programming in real-life examples. The book starts off with an introduction to AWK essentials. You will then be introduced to regular expressions, AWK variables and constants, arrays and AWK functions and more. The book then delves deeper into more complex tasks, such as printing formatted output in AWK, control flow statements, GNU's implementation of AWK covering the advanced features of GNU AWK, such as network communication, debugging, and inter-process communication in the GAWK programming language which is not easily possible with AWK. By the end of this book, the reader will have worked on the practical implementation of text processing and pattern matching using AWK to perform routine tasks.

Who is this book for?

This book is for developers or analysts who are inclined to learn how to do text processing and data extraction in a Unix-like environment. Basic understanding of Linux operating system and shell scripting will help you to get the most out of the book.

What you will learn

  • -Create and use different expressions and control flow statements in AWK
  • -Use Regular Expressions with AWK for effective text-processing
  • -Use built-in and user-defined variables to write AWK programs
  • -Use redirections in AWK programs and create structured reports
  • -Handle non-decimal input, 2-way inter-process communication with Gawk
  • -Create small scripts to reformat data to match patterns and process texts
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Table of Contents

10 Chapters
Getting Started with AWK Programming Chevron down icon Chevron up icon
Working with Regular Expressions Chevron down icon Chevron up icon
AWK Variables and Constants Chevron down icon Chevron up icon
Working with Arrays in AWK Chevron down icon Chevron up icon
Printing Output in AWK Chevron down icon Chevron up icon
AWK Expressions Chevron down icon Chevron up icon
AWK Control Flow Statements Chevron down icon Chevron up icon
AWK Functions Chevron down icon Chevron up icon
GNU's Implementation of AWK – GAWK (GNU AWK) Chevron down icon Chevron up icon
Practical Implementation of AWK Chevron down icon Chevron up icon

Customer reviews

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Robert Bednarczyk Feb 19, 2024
Full star icon Full star icon Full star icon Full star icon Full star icon 5
The awk language is a much-needed programming language for managing linux systems. The book teaches very well how to use this language correctly and effectively.
Feefo Verified review Feefo
Amazon Customer Jun 22, 2018
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This book is seriously amazing! You will be up and programming with AWK in no time. Very Very good book - esp. for Linux Systems Administrators lol
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software_craftsman Jun 12, 2019
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This is the AWK Book that I have been waiting to read for many years. I finally feel that I have mastered Awk Programming instead of whipping up little one liners. This will teach you many concepts about the shell as well.
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Dr Rick Dec 06, 2020
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A very thorough and gentle way to get up to speed with Awk.
Amazon Verified review Amazon
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