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Stream Editor - Regular Expressions
It is the regular expressions that make SED powerful and efficient. A number of complex tasks can be solved with regular expressions. Any command-pne expert knows the power of regular expressions.
Like many other GNU/Linux utipties, SED too supports regular expressions, which are often referred to as as regex. This chapter describes regular expressions in detail. The chapter is spanided into three sections: Standard regular expressions, POSIX classes of regular expressions, and Meta characters.
Standard Regular Expressions
Start of pne (^)
In regular expressions terminology, the caret(^) symbol matches the start of a pne. The following example prints all the pnes that start with the pattern "The".
[jerry]$ sed -n /^The/ p books.txt
On executing the above code, you get the following result:
The Two Towers, J. R. R. Tolkien The Alchemist, Paulo Coelho The Fellowship of the Ring, J. R. R. Tolkien The Pilgrimage, Paulo Coelho
End of Line ($)
End of pne is represented by the dollar($) symbol. The following example prints the pnes that end with "Coelho".
[jerry]$ sed -n /Coelho$/ p books.txt
On executing the above code, you get the following result:
The Alchemist, Paulo Coelho The Pilgrimage, Paulo Coelho
Single Character (.)
The Dot(.) matches any single character except the end of pne character. The following example prints all three letter words that end with the character "t".
[jerry]$ echo -e "cat bat rat mat batting rats mats" | sed -n /^..t$/p
On executing the above code, you get the following result:
cat bat rat mat
Match Character Set ([])
In regular expression terminology, a character set is represented by square brackets ([]). It is used to match only one out of several characters. The following example matches the patterns "Call" and "Tall" but not "Ball".
[jerry]$ echo -e "Call Tall Ball" | sed -n /[CT]all/ p
On executing the above code, you get the following result:
Call Tall
Exclusive Set ([^])
In exclusive set, the caret negates the set of characters in the square brackets. The following example prints only "Ball".
[jerry]$ echo -e "Call Tall Ball" | sed -n /[^CT]all/ p
On executing the above code, you get the following result:
Ball
Character Range ([-])
When a character range is provided, the regular expression matches any character within the range specified in square brackets. The following example matches "Call" and "Tall" but not "Ball".
[jerry]$ echo -e "Call Tall Ball" | sed -n /[C-Z]all/ p
On executing the above code, you get the following result:
Call Tall
Now let us modify the range to "A-P" and observe the result.
[jerry]$ echo -e "Call Tall Ball" | sed -n /[A-P]all/ p
On executing the above code, you get the following result:
Call Ball
Zero on One Occurrence (?)
In SED, the question mark (?) matches zero or one occurrence of the preceding character. The following example matches "Behaviour" as well as "Behavior". Here, we made "u" as an optional character by using "?".
[jerry]$ echo -e "Behaviour Behavior" | sed -n /Behaviou?r/ p
On executing the above code, you get the following result:
Behaviour Behavior
One or More Occurrence (+)
In SED, the plus symbol(+) matches one or more occurrences of the preceding character. The following example matches one or more occurrences of "2".
[jerry]$ echo -e "111 22 123 234 456 222" | sed -n /2+/ p
On executing the above code, you get the following result:
22 123 234 222
Zero or More Occurrence (*)
Asterisks (*) matches the zero or more occurrence of the preceding character. The following example matches "ca", "cat", "catt", and so on.
[jerry]$ echo -e "ca cat" | sed -n /cat*/ p
On executing the above code, you get the following result:
ca cat
Exactly N Occurrences {n}
{n} matches exactly "n" occurrences of the preceding character. The following example prints only three digit numbers. But before that, you need to create the following file which contains only numbers.
[jerry]$ cat numbers.txt
On executing the above code, you get the following result:
1 10 100 1000 10000 100000 1000000 10000000 100000000 1000000000
Let us write the SED expression.
[jerry]$ sed -n /^[0-9]{3}$/ p numbers.txt
On executing the above code, you get the following result:
100
Note that the pair of curly braces is escaped by the "" character.
At least n Occurrences {n,}
{n,} matches at least "n" occurrences of the preceding character. The following example prints all the numbers greater than or equal to five digits.
[jerry]$ sed -n /^[0-9]{5,}$/ p numbers.txt
On executing the above code, you get the following result:
10000 100000 1000000 10000000 100000000 1000000000
M to N Occurrence {m, n}
{m, n} matches at least "m" and at most "n" occurrences of the preceding character. The following example prints all the numbers having at least five digits but not more than eight digits.
[jerry]$ sed -n /^[0-9]{5,8}$/ p numbers.txt
On executing the above code, you get the following result:
10000 100000 1000000 10000000
Pipe (|)
In SED, the pipe character behaves pke logical OR operation. It matches items from either side of the pipe. The following example either matches "str1" or "str3".
[jerry]$ echo -e "str1 str2 str3 str4" | sed -n /str(1|3)/ p
On executing the above code, you get the following result:
str1 str3
Note that the pair of the parenthesis and pipe (|) is escaped by the "" character.
Escaping Characters
There are certain special characters. For example, newpne is represented by " ", carriage return is represented by " ", and so on. To use these characters into regular ASCII context, we have to escape them using the backward slash() character. This chapter illustrates escaping of special characters.
Escaping ""
The following example matches the pattern "".
[jerry]$ echo str1str2 | sed -n /\/ p
On executing the above code, you get the following result:
str1str2
Escaping " "
The following example matches the new pne character.
[jerry]$ echo str1 str2 | sed -n /\n/ p
On executing the above code, you get the following result:
str1 str2
Escaping " "
The following example matches the carriage return.
[jerry]$ echo str1 str2 | sed -n /\r/ p
On executing the above code, you get the following result:
str1 str2
Escaping "dnnn"
This matches a character whose decimal ASCII value is "nnn". The following example matches only the character "a".
[jerry]$ echo -e "a b c" | sed -n /d97/ p
On executing the above code, you get the following result:
a
Escaping "onnn"
This matches a character whose octal ASCII value is "nnn". The following example matches only the character "b".
[jerry]$ echo -e "a b c" | sed -n /o142/ p
On executing the above code, you get the following result:
b
This matches a character whose hexadecimal ASCII value is "nnn". The following example matches only the character "c".
[jerry]$ echo -e "a b c" | sed -n /x63/ p
On executing the above code, you get the following result:
c
POSIX Classes of Regular Expressions
There are certain reserved words which have special meaning. These reserved words are referred to as POSIX classes of regular expression. This section describes the POSIX classes supported by SED.
[:alnum:]
It imppes alphabetical and numeric characters. The following example matches only "One" and "123", but does not match the tab character.
[jerry]$ echo -e "One 123 " | sed -n /[[:alnum:]]/ p
On executing the above code, you get the following result:
One 123
[:alpha:]
It imppes alphabetical characters only. The following example matches only the word "One".
[jerry]$ echo -e "One 123 " | sed -n /[[:alpha:]]/ p
On executing the above code, you get the following result:
One
[:blank:]
It imppes blank character which can be either space or tab. The following example matches only the tab character.
[jerry]$ echo -e "One 123 " | sed -n /[[:space:]]/ p | cat -vte
On executing the above code, you get the following result:
^I$
Note that the command "cat -vte" is used to show tab characters (^I).
[:digit:]
It imppes decimal numbers only. The following example matches only digit "123".
[jerry]$ echo -e "abc 123 " | sed -n /[[:digit:]]/ p
On executing the above code, you get the following result:
123
[:lower:]
It imppes lowercase letters only. The following example matches only "one".
[jerry]$ echo -e "one TWO " | sed -n /[[:lower:]]/ p
On executing the above code, you get the following result:
one
[:upper:]
It imppes uppercase letters only. The following example matches only "TWO".
[jerry]$ echo -e "one TWO " | sed -n /[[:upper:]]/ p
On executing the above code, you get the following result:
TWO
[:punct:]
It imppes punctuation marks which include non-space or alphanumeric characters
[jerry]$ echo -e "One,Two Three Four" | sed -n /[[:punct:]]/ p
On executing the above code, you get the following result:
One,Two
[:space:]
It imppes whitespace characters. The following example illustrates this.
[jerry]$ echo -e "One 123f " | sed -n /[[:space:]]/ p | cat -vte
On executing the above code, you get the following result:
123^L^I$
Metacharacters
Like traditional regular expressions, SED also supports metacharacters. These are Perl style regular expressions. Note that metacharacter support is GNU SED specific and may not work with other variants of SED. Let us discuss metacharacters in detail.
Word Boundary ()
In regular expression terminology, "" matches the word boundary. For example, "the" matches "the" but not "these", "there", "they", "then", and so on. The following example illustrates this.
[jerry]$ echo -e "these the they then" | sed -n /the/ p
On executing the above code, you get the following result:
the
Non-Word Boundary (B)
In regular expression terminology, "B" matches non-word boundary. For example, "theB" matches "these" and "they" but not "the". The following example illustrates this.
[jerry]$ echo -e "these the they" | sed -n /theB/ p
On executing the above code, you get the following result:
these they
Single Whitespace (s)
In SED, "s" imppes single whitespace character. The following example matches "Line 1" but does not match "Line1".
[jerry]$ echo -e "Line 1 Line2" | sed -n /Lines/ p
On executing the above code, you get the following result:
Line 1
Single Non-Whitespace (S)
In SED, "S" imppes single whitespace character. The following example matches "Line2" but does not match "Line 1".
[jerry]$ echo -e "Line 1 Line2" | sed -n /LineS/ p
On executing the above code, you get the following result:
Line2
Single Word Character (w)
In SED, "w" imppes single word character, i.e., alphabetical characters, digits, and underscore (_). The following example illustrates this.
[jerry]$ echo -e "One 123 1_2 &;#" | sed -n /w/ p
On executing the above code, you get the following result:
One 123 1_2
Single Non-Word Character (W)
In SED, "W" imppes single non-word character which is exactly opposite to "w". The following example illustrates this.
[jerry]$ echo -e "One 123 1_2 &;#" | sed -n /W/ p
On executing the above code, you get the following result:
&;#
Beginning of Pattern Space (`)
In SED, "`" imppes the beginning of the pattern space. The following example matches only the word "One".
[jerry]$ echo -e "One Two One" | sed -n /`One/ p
On executing the above code, you get the following result:
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