How to Find and Replace Complex Strings by Using Sed With Regex

1. Find and Replace Strings Using sed in Bash
2. Find and Replace the Delimiter Character / in a String in Bash
3. Find All the Numbers and Replace Them With the String Number in Bash

When working with text files, you’ll frequently need to identify and replace text strings in one or more files.

sed is a stream editor. It can perform rudimentary text manipulation on files and input streams such as pipelines. You can use sed to search for, find, and replace words and lines, as well as insert and delete them.

Basic and extended regex is supported, allowing you to match complex patterns. In this post, we’ll look at how to use sed to find and replace strings. We’ll also demonstrate how to use recursive search and replace.

Find and Replace Strings Using sed in Bash

There are various versions of sed, each with its features.

Linux distributions come with GNU sed pre-installed by default, but Mac OS utilizes the BSD version. We’re going to use the GNU version.

The syntax is used for searching and replacing text with sed:

sed -e 's/Search_Regex/substitution/g' Filename

In the above syntax used, the following are the definition:

• The -e is sed’s output written to the standard output by default. This parameter instructs sed to edit files in their current location. If an extension is defined, a replacement file is generated.
• The /// is a character that acts as a separator. Although most people use the slash (/) character, it can be any character.
• The Search_Regex is a to search for something. We will be using a regex or a regular string.
• The substitution is used as strings to substitute.
• The g is a flag for global substitution. The sed reads the file line by line by default and only alters the first occurrence of the Search_Regex on each line. All instances are replaced when the replacement flag is set.
• The Filename is the file’s name that should run the command.

Let’s look at the commonly used parameters and flags for using the sed command to search and replace text in files. We will use the example file we have created for demonstration purposes, i.e., live.txt.

live.txt 
Omicron is the endgame of Covid and Omicron 9876

If the g flag is ignored, only the first instance of the search string in each line is replaced.

$ sed -e 's/endgame/infinitywar/' live.txt

Output:

Omicron is the infinitywar of Covid and Omicron 9876

The sed replaces all occurrences of the search pattern with the global replacement flag:

$ sed -e 's/Omicron/thanos/g' live.txt

Output:

thanos is the endgame of Covid and thanos 9876

As you may have observed, the string Omicron has been substituted with thanos.

Find and Replace the Delimiter Character / in a String in Bash

You must use the backslash \ to escape the slash if you want to discover and replace a string that contains the delimiter character /. For example, to replace /bin/bash with /usr/bin/zsh, use the following:

$ sed -e 's/\/bin\/bash/\/usr\/bin\/zsh/g' live.txt

Output:

Omicron is the endgame of Covid and Omicron 9876

Let’s make it easier to understand using a vertical bar | or a colon :, although any other character will suffice.

$ sed -e 's|/bin/bash|/usr/bin/zsh|g' live.txt

Output:

Omicron is the endgame of Covid and Omicron 9876

As you can see, the result is identical to the prior one.

Find All the Numbers and Replace Them With the String Number in Bash

Regular expressions can also find all four numbers and replace them with the string number. For example:

$ sed -e 's/\b[0-9]\{4\}\b/number/g' live.txt

Output:

Omicron is the endgame of Covid, and Omicron number

As you can see, the 9876 is replaced by the number string.

Last but not least, whenever you use sed to edit a file, you should always make a backup. Give the -i option an extension for the backup file to accomplish this.

For example, edit live.txt and save the original file as live.txt. If you were to utilize bak, you’d do that by:

$ sed -i.bak 's/Omicron/thanos/g' live.txt

List the files with the ls command to ensure that the backup was created:

$ ls

Output:

live.txt
live.txt.bak 

Although it may appear complicated at first, finding and changing the text in files with sed is relatively straightforward.