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🦑🐧5 best CLI tools for finding words using regular expressions:

1) Grep command
First of all comes the utility tool grep, which stands for Global Regular Expression Print, a powerful command line tool that comes in handy when searching for a specific string or pattern in a file.

Grep comes by default with modern Linux distributions and gives you the ability to return different search results.

With grep, you can perform a wide variety of functions such as:

Search for strings or matching patterns in the file.
Search for strings or matching patterns in Gzip files.
Count the number of string matches.
Print line numbers containing a string or pattern.
Recursive search for a string in directories.
Perform a reverse search (that is, display results for rows that do not match your search criteria).
Ignore case sensitivity when searching for strings.
The syntax for using the grep command is pretty simple:

$ grep pattern FILE
For example, to find the string "itsecforu" in a file, say hello.txt, ignoring case, run the command:
$ grep -i itsecforu hello.txt

2) sed command
Sed, short for Stream Editor, is another useful command line tool for manipulating text in a text file.

Sed searches, filters, and replaces lines in a given file in a non-interactive way.

By default, the sed command prints the output to STDOUT (standard output), implying that the output is written to the terminal rather than saved to a file.

The Sed command is invoked as follows:

$ sed -OPTIONS command [file]
For example, to replace all instances of "Unix" with "Linux", issue the command:
$ sed 's / Unix / Linux' hello.txt
If you want to redirect the output rather than display it on the terminal, use the redirection sign (>) as follows:
$ sed 's / Unix / Linux' hello.txt> output.txt
The output of the command will be saved in the file output.txt, rather than being printed to the screen.

Check the man pages again for other options that you can use.

$ man sed

3) Team Ack
Ack is a fast and portable command line tool written in Perl.

Ack is considered a convenient replacement for grep, and displays results in a visually appealing form.

The Ack command searches a file or directory for strings that match the search criteria.

Then it highlights the corresponding line.

Ack has the ability to differentiate between files based on their extensions and, to a certain extent, the content in the files.

Ack command syntax:

$ ack [options] PATTERN [FILE ...]
$ ack -f [options] [DIRECTORY ...]
For example, to find the word itsecforu, run:

$ ack itsecforu hello.txt
To install ask on your system, run the command:

$ sudo apt install ack-grep [On Debian / Ubuntu]
$ sudo dnf install ack-grep [On CentOS / RHEL]

4) Team Awk
Awk is a complete scripting language, word processing and data manipulation tool.

It looks for files or programs that contain a search pattern.

When a string or pattern is found, awk performs the action on the match or string and prints the results to STDOUT.

The AWK pattern is enclosed in curly braces, and the entire program is enclosed in single quotes.

Let's take the simplest example.

Suppose you are displaying your system date as shown below:

$ date

Suppose you only want to display the first value, that is, the day of the week.
In this case, pipe the output to awk as shown below:
$ date | awk '{print $ 1}'
To display subsequent values, separate them with a comma, as shown below:
$ date | awk '{print $ 1, $ 2}'
The above command will display the day of the week and date .

5) Silver Searcher
Silver Searcher is a cross-platform open source code search tool similar to ack but with a focus on speed.

It makes it easy to find a specific string in files in no time at all:

$ ag OPTIONS search_pattern / path / to / file
For example, to find the string "Linux" in hello.txt, run the command:
$ ag Linux hello.txt
For additional options:

$ man ag

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