Regex for Linux, regular expressions are similar to the wildcards which allow us to create a pattern to perform a specific search in data. Regular expressions are lot more powerful and also known as “regex” or “regexp“.
The commands in Linux that are commonly used with regex are tr, sed, vi and grep.
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Basic Regular Expressions
Here are listed some of the basic regular expressions used on Linux.
| Expression | Description |
|---|---|
| ^ | Matches the beginning of the string |
| $ | Matches the end of the string |
| . | Matches any single character except “n“ |
| * | Matches up zero or more times the preceding character |
| \ | Preserve the meaning of the character by escaping it |
| () | Group the characters |
| ? | Match one character |
Examples
To search a file with a specific letter you can use the following command.
grep a content.txt
This command searches for the occurrence of “a” in the content.txt file and outputs it.
To search for a line that starts with a specific character or word you can use this “^” expression.
grep ^d content.txt
This command outputs all lines that is starting with “d”.
To search for a line that ends with a specific character or word you can use this “$” expression.
grep g$ content.txt
This command outputs all lines that is ending with “g”.
To search for empty line you can use the following command.
grep '^$' content.txt
Interval Regular Expressions
The interval regular expressions are used to match the number of times the characters appear in a string.
| Expression | Description |
|---|---|
| {n} | Matches the character appearing “n” number of times. |
| {n,m} | Matches the character appearing “n” number of times but not more than “m“ |
| {n,} | Matches only if there are more than “n” number of characters |
Example
grep -E n{2} content.txt
This command outputs the line that contains 2 occurrences of “n”.
Output
Running
Extended Regular Expressions
Extended regular expressions are combination of two or more expressions as listed below.
| Expression | Description |
|---|---|
| \+ | Match one or more occurrences of the previous character. |
| \? | Matches zero or one occurrence of the previous character. |
Example
To filter out lines where character ‘o’ precedes character ‘m’ you can use this kind of expression.
grep "o\+m" content.txt
This command outputs something like this.
Output
Zombies
These are some of the common regular expressions used in Linux.
Conclusion
Now you have learned how to create a regular expression for Linux.
Thanks for your time. If you face any problem or any feedback, please leave a comment below.
