Regular expressions overview
Regular expressions are a defined rules, Linux tools can use to filter text.
Basic regular expressions
Plain Text
[root@node1 ~]# echo "this is a cat" | sed -n '/cat/p'
this is a cat
[root@node1 ~]# echo "this is a cat" | gawk '/cat/{print $0}'
this is a cat
Regular expression matching is very picky, especially need to remember that regular expressions are case-sensitive.
Special characters
Special characters in regular expressions identified include:
.*[]^${}\+?|()
If you want to use a special character as a text character, it must be escaped, general use (\) to escape.
[root@node1 ~]# echo "this is a $" | sed -n '/\$/p' this is a $
Anchor Character
There are two special characters may be used to lock the pattern in the first row or end of data stream
The caret (^) define the mode from the beginning of the line data stream Chinese Bank began.
The dollar sign ($) defines the end of the line anchor.
[root@node1 ~]# echo "this is a cat" | sed -n '/^this/p' this is a cat [root@node1 ~]# echo "this is a cat" | sed -n '/cat$/p' this is a cat
In some cases these two commands may be used in combination
1. For example, find the line that contains only specific text
[root@node1 ljy]# more test.txt this is a dog what how this is a cat is a dog [root@node1 ljy]# sed -n '/^is a dog$/p' test.txt is a dog [root@node
2. The combination of two anchors, blank lines may be directly filtered
[root@node1 ljy]# more test.txt this is a dog what how this is a cat is a dog [root@node1 ljy]# sed '/^$/d' test.txt this is a dog what how this is a cat is a dog
Dot character
Dot matches any other single newline character, he must match a character.
[root@node1 ljy]# more test.txt this is a dog what how this is a cat is a dog at [root@node1 ljy]# sed -n '/.at/p' test.txt what this is a cat
Character Group
DETAILED defined characters to be matched using a set of characters. Use square brackets to define a character group.
[root@node1 ljy]# more test.txt this is a dog this is a Dog this is a DoG this is a cat [root@node1 ljy]# sed -n '/[dD]og/p' test.txt this is a dog this is a Dog [root@node1 ljy]# sed -n '/[dD]o[gG]/p' test.txt this is a dog this is a Dog this is a DoG
Negated character set
To exclude certain elements, to add a caret in front of the character set.
[root@node1 ljy]# sed -n '/[dD]o[gG]/p' test.txt this is a dog this is a Dog this is a DoG [root@node1 ljy]# sed -n '/[^D]og/p' test.txt this is a dog
Interval
Regular expressions can include any character in this range.
[root@node1 ljy]# more test.txt 123123 1231 121222222 412345341613 vsdvs qwer12344123 12345 34211 444444 [root@node1 ljy]# sed -n '/^[0-9][0-9][0-9][0-9][0-9]$/p' test.txt 12345 34211
Expand the regular expression
question mark
Question mark indicates that the preceding character appear 0 or 1, limited.
[root@node1 ljy]# echo "bat" | gawk '/ba?t/{print $0}'
bat
[root@node1 ljy]# echo "baat" | gawk '/ba?t/{print $0}'
[root@node1 ljy]# echo "bt" | gawk '/ba?t/{print $0}'
bt
Can be used with a question mark and character set
[root@node1 ljy]# echo "bt" | gawk '/b[ae]?t/{print $0}'
bt
[root@node1 ljy]# echo "bat" | gawk '/b[ae]?t/{print $0}'
bat
[root@node1 ljy]# echo "bet" | gawk '/b[ae]?t/{print $0}'
bet
[root@node1 ljy]# echo "baat" | gawk '/b[ae]?t/{print $0}'
plus
Plus sign indicates that the preceding character may occur once or more times, but at least once.
[root@node1 ljy]# echo "baat" | gawk '/b[ae]+t/{print $0}'
baat
[root@node1 ljy]# echo "bt" | gawk '/b[ae]+t/{print $0}'
[root@node1 ljy]# echo "bt" | gawk '/ba+t/{print $0}'
[root@node1 ljy]# echo "bat" | gawk '/ba+t/{print $0}'
bat
[root@node1 ljy]# echo "baat" | gawk '/ba+t/{print $0}'
baat
curly braces
ERE in curly braces allows you to repeat regular expression specified limits.
m, n m This minimum occurs, occurs most n.
[root@node1 ljy]# echo "baat" | gawk '/b[ae]{1,2}t/{print $0}'
baat
[root@node1 ljy]# echo "baaat" | gawk '/b[ae]{1,2}t/{print $0}'
The pipe symbol
Specified regular expression rules or logical manner in which to comply with a condition it can be.
Expressions grouping
Regular expression grouping can also be grouped with parentheses.
[root@node1 ljy]# echo "bat" | gawk '/b(a|e)t/{print $0}'
bat
[root@node1 ljy]# echo "baat" | gawk '/b(a|e)t/{print $0}'
[root@node1 ljy]# echo "bet" | gawk '/b(a|e)t/{print $0}'
bet