Regular Expression (Regex, and Regexp) in Linux Ft. Grep

A regular expression (also known as regex, and regexp) is a sequence of characters used by various programming languages such as python, Linux tools such as grep, awk, sed, etc. to match one or more strings.

For instance, the regex “R.*x” matches with strings “Regex“, and “Regular Expression in Linux“, etc.

Despite being super useful, there is not a single standard that is followed everywhere. For example, we have GNU Regex, POSIX Regex, Perl Regex, etc.

However, there are very few variations across these standards. For instance, sed in Linux and Mac OS follow standards slightly different from each other. Here, in this article, I will be focusing mainly on the GNU Regex using grep command in Linux. At the same time, I will also be mentioning these variations which come to my mind.

Table of Contents

• Regex Is Not a Shell Pattern
• Special and Ordinary Characters in Regular Expression
  • Ordinary Characters
  • Special Characters
• Dot (.) in Regular Expression
• Caret (^) in Regex
• Dollar ($) in Regex
• Character Class: Bracket Expression ([ ]) in Regexp
• Named Class (aka Named Set) in Regex
• Backslash Based Regex (\b, \B, \w, \W, \s, \S, \>, \<)
• Repetition Operators in Regular Expression (*, +, ?, {n,m})
• Alternation (or Infix) Operator (|) in Regexp
• Grouping in Regex
• Back-references in Regexp
• Conclusion

Regex Is Not a Shell Pattern

I need to warn you that regexp is completely different from the zsh or bash’s glob (also called shell-pattern).

For example, *.mp4 in shell pattern means any filename ending with .mp4. On the other hand, the star (*) at the start of a regex is a null character.

Similarly, ?, () and | have different meanings.

At the same time, you need to prevent the shell from interpreting your regex as a shell pattern. For this, you need to surround it with double quotes (preferably single quotes).

Special and Ordinary Characters in Regular Expression

Ordinary Characters

Any character other than the .?*+{|()[\^$ are called ordinary characters and they are interpreted as they are.

For the example given below, the regex linux is made of ordinary characters and hence it is interpreted as it is.

Note: By the way, If a line does not contain the given regex pattern, grep does not print that line. Therefore, in the above example, the first line is omitted.

Special Characters

The characters .?*+{|()[\^$ are called special characters (also known as metacharacters) since they have special meanings in the regex:

Regular expression	Meaning
.	any single character
*	the preceding item matching zero or more times
+	the preceding item matching one or more times
^	beginning of the line
$	end of the line
?	the preceding item is optional
[	list of characters, range of characters, named classes
{	used for interval expression
|	the infix Operator (the OR Alternate Operator)
(	used for grouping
\	have meanings in combination with other characters; ex- \b, \<, \w, \, etc.

Although these characters are special characters, you can force your regex engine to treat them as ordinary characters by prepending them with a backslash. Example –

Now I will be explaining all these special characters in the upcoming headings with examples.

Dot (.) in Regular Expression

Dot will match any single character.

In the following example, only “fix” from the first line is matched. Here, “i” is equated as the dot. In the second line, there is no such character between f and x and hence, nothing is matched in the second line.

Caret (^) in Regex

The caret (^) means to match an empty string at the start of a line.

For example, the following command searches for the “linux” at the beginning. Since the second line does not have “linux” at the start, hence that is not printed.

Dollar ($) in Regex

The dollar is used to match an empty string at the end of a line. For the following example, only the second line is matched because only that one has “linux” at the end.

Note: The charet (^) and dollar ($) are also called regex anchors.

Character Class: Bracket Expression ([ ]) in Regexp

Character Class (also known as Character Set) is a list of characters in the [] to match any one character from the list. There are many types:

[az]	the character “a” OR “z”
[a-z]	any letter from a to z (lowercase)
[A-Z]	any letter from A to Z (uppercase)
[A-Za-z]	any letter
[0-9]	any number
[-az]	any one character out of the three -, a, z
[^abc]	negates [abc] i.e. matches any character except a, b, c (called Negated Character Class)

Note 1: Letters and numbers have different meanings in different countries and languages. The above table is for the traditional C locale. In simple words, if your work is based on English then it should work fine.

Explanations with examples:

• In the following example, regex i[sn] matches either is or in.

• The dash (-) is used for “Range Expression”. In the example given below, the [0-9] matches with only the digits..

• [A-Za-z] means any alphabetic character as shown below:

• To include the literal dash in your list as well, put the dash at the beginning in the bracket.

• Negated Character Class: Continuing with the above example, [^-10] just matches any character other than the -, 1, 0.

The above-mentioned Character Classes are sufficient to create any list. But before creating any convoluted list such as [A-Za-z0-9], you can also use also corresponding Named Classes explained below.

Named Class (aka Named Set) in Regex

Named Classes are predefined character classes. For example, [:alpha:] equals to any upper or lowercase alphabet

Here is a list of widely used named classes (source: man grep, man gawk):

• [:lower:] – lowercase letters
• [:upper:] – uppercase letters
• [:alpha:] – alphabets
• [:digit:] – digits
• [:alnum:] – alphabets or numbers i.e. [A-Za-z0-9]
• [:punct:] – punctuation characters (characters that are not letter, digits, control characters, or space characters)
• [:space:] – any space character (space, horizontal and vertical tabs, newline, carriage return, and formfeed).
• [:blank:] – space or tab
• [:cntrl:] – control characters
• [:print:] – printable characters i.e. [:punct:],[:alnum:], space
• [:graph:] – graphical characters i.e. [:alnum:] and [:punct:]
• [:xdigit:]– hexadecimal digits

Note 1: Special Characters lose their special meanings in the bracket. However, some of them can get it back by their special placement in the bracket. For instance, you need to place ^ anywhere but first, dash (-) at first, and ] at the first position to get back their special meanings.

Note 2: The Named Classes can be included with other regular expressions in the brackets. For instance, [[:upper:][:lower:]] equals both upper and lower alphabets.

Backslash Based Regex (\b, \B, \w, \W, \s, \S, \>, \<)

\w is a word-constituent character (letter, digit, or underscore). Anything other than that is \W (non-word-constituent character). Ex –

\s is [[:space:]] mentioned above. \S (non-whitespace) is the exact opposite.

\b equals to an empty string at the edge of a word (source: grep’s manual). In GNU gawk command of Linux, it is \y.

\B matches to an empty string provided it’s not at the edge of a word. (source: man grep)

\< and \> match the empty string at the beginning and end of a word (respectively).

Repetition Operators in Regular Expression (*, +, ?, {n,m})

Regular expression	Meaning
?	the preceding item is optional
*	the preceding item matching zero or more times
+	the preceding item matching one or more times
{n}	the preceding item matching exactly n times
{n,}	the preceding item matching n or more times
{,m}	the preceding item matching m or less than m times
{n,m}	the preceding item matching n to m times

Now I will describe them with examples.

• Star (*): In the following example, “f.*x” means “f, then any character (.) repeated zero or more times (*), and then x”.

• Plus (+): Similarly, “f.+x” means “f, then any character (.) repeated one or more times (+), and then x”.

• Question Mark (?) in regex makes previous item optional. Therefore, in the following example, https? matches with both http and https:

• {n}: For the following example, .{5} means any character (dot) repeated 5 times. Therefore, first grep prints any 5 character. Then it prints another 5 character from the remaining part. And then again it prints the next 5 character. Now, the text does not have 5 character left and hence nothing is printed.

• {n,},{,m},{n,m}, similarly, means the preceding item repeated n or more, m or less, n to m times respectively.

Alternation (or Infix) Operator (|) in Regexp

When you insert an Infix Operator | between two regular expressions, the regex engine chooses one of the two expressions.

Grouping in Regex

You can put a group of characters in parenthesis and force the regex engine to treat them as a unit.

For instance, in the following example, a group of characters smart is repeated twice. On the other hand, regex smart2 means smartt.

Back-references in Regexp

Back-references in a regex are denoted by \n, where n is a number and they “refer back” to the nth parenthesized regex.

For example given below, \2 refers to the second parenthesized part i.e. tech and \1 to the first parenthesized part i.e. smart. In other words, \2 is replaced with the second one and \1 with the first.

Conclusion

That’s all folks. If you want to revise it quickly, just have a quick look at the given tables.

Thanks for reading and if you have any suggestions/problems, please comment below.