Topic 5
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Topic 5
REGULAR EXPRESSIONS
TOPIC 5 – REGULAR EXPRESSIONS 1
Regular Expression
A pattern of special characters used to match strings in a search
Typically made up from special characters called metacharacters
Regular expressions are used thoughout UNIX:
◦ Editors: ed, ex, vi
◦ Utilities: grep, egrep, sed, and awk
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Metacharacters
any non-metacharacter matches itself
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RE Metacharacter Matches…
. Any one character, except new line
[a-z] Any one of the enclosed characters (e.g. a-z)
* Zero or more of preceding character
? or \? Zero or one of the preceding characters
+ or \+ One or more of the preceding characters
The grep Utility
“grep” command:
searches for text in file(s)
Examples:
% grep root mail.log
% grep r..t mail.log
% grep ro*t mail.log
% grep ‘ro*t’ mail.log
% grep ‘r[a-z]*t’ mail.log
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more Metacharacters
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RE Metacharacter Matches…
^ beginning of line
$ end of line
\char Escape the meaning of char following it
[^] One character not in the set
\< Beginning of word anchor \> End of word anchor
( ) or \( \) Tags matched characters to be used later (max = 9)
| or \| Or grouping
x\{m\} Repetition of character x, m times (x,m = integer)
x\{m,\} Repetition of character x, at least m times
x\{m,n\} Repetition of character x between m and m times
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Regular Expression
An atom specifies what text is to be matched and
where it is to be found.
An operator combines regular expression atoms.
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Atoms
An atom specifies what text is to be matched and where
it is to be found.
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Single-Character Atom
A single character matches itself
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Dot Atom
matches any single character except for a new
line character (\n)
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Class Atom
matches only single character that can be any of
the characters defined in a set:
Example: [ABC] matches either A, B, or C.
Notes:
1) A range of characters is indicated by a dash, e.g. [A-Q]
2) Can specify characters to be excluded from the set, e.g.
[^0-9] matches any character other than a number.
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Example: Classes
short-hand classes
[:alnum:]
[:alpha:]
[:upper:]
[:lower:]
[:digit:]
[:space:]
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TOPIC 5 – REGULAR EXPRESSIONS 13
Anchors
Anchors tell where the next character in the pattern must
be located in the text data.
Back References: \n
used to retrieve saved text in one of nine buffers
can refer to the text in a saved buffer by using a back reference:
ex.: \1 \2 \3 …\9
more details on this later
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TOPIC 5 – REGULAR EXPRESSIONS 15
Operators
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Sequence Operator
In a sequence operator, if a series of atoms are shown in
a regular expression, there is no operator between them.
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Alternation Operator: | or \|
operator (| or \| ) is used to define one
or more alternatives
Note: depends on version of “grep”
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Repetition Operator: \{…\}
The repetition operator specifies that the atom or
expression immediately before the repetition may be
repeated.
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Basic Repetition Forms
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Short Form Repetition Operators: * + ?
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Group Operator
In the group operator, when a group of characters is
enclosed in parentheses, the next operator applies to the
whole group, not only the previous characters.
Note: depends on version of “grep”
use \( and \) instead
Grep detail and examples
grep is family of commands
◦ grep
common version
◦ egrep
understands extended REs
(| + ? ( ) don’t need backslash)
◦ fgrep
understands only fixed strings, i.e. is faster
◦ rgrep
will traverse sub-directories recursively
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Commonly used “grep” options:
-c Print only a count of matched lines.
-i Ignore uppercase and lowercase distinctions.
-l List all files that contain the specified pattern.
-n Print matched lines and line numbers.
-s Work silently; display nothing except error messages.
Useful for checking the exit status.
-v Print lines that do not match the pattern.
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Example: grep with pipe
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% ls -l | grep ‘^d’
drwxr-xr-x 2 krush csci 512 Feb 8 22:12 assignments
drwxr-xr-x 2 krush csci 512 Feb 5 07:43 feb3
drwxr-xr-x 2 krush csci 512 Feb 5 14:48 feb5
drwxr-xr-x 2 krush csci 512 Dec 18 14:29 grades
drwxr-xr-x 2 krush csci 512 Jan 18 13:41 jan13
drwxr-xr-x 2 krush csci 512 Jan 18 13:17 jan15
drwxr-xr-x 2 krush csci 512 Jan 18 13:43 jan20
drwxr-xr-x 2 krush csci 512 Jan 24 19:37 jan22
drwxr-xr-x 4 krush csci 512 Jan 30 17:00 jan27
drwxr-xr-x 2 krush csci 512 Jan 29 15:03 jan29
% ls -l | grep -c ‘^d’
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Pipe the output of the
“ls –l” command to
grep and list/select
only directory entries.
Display the number of
lines where the pattern
was found. This does
not mean the number
of occurrences of the
pattern.
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Example: grep with \< \>
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% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
% grep ‘\
north NO Ann Stephens 455000.50
Print the line if it contains the word “north”.
Example: grep with a\|b
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% grep ‘NW\|EA’ grep-datafile
northwest NW Charles Main 300000.00
eastern EA TB Savage 440500.45
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Print the lines that contain either the expression “NW” or the expression “EA”
Note: egrep works with |
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Example: egrep with +
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% egrep ‘3+’ grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
Print all lines containing one or more 3’s.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Note: grep works with \+
Example: egrep with RE: ?
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% egrep ‘2\.?[0-9]’ grep-datafile
southwest SW Lewis Dalsass 290000.73
Print all lines containing a 2, followed by zero or one period, followed by a number.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Note: grep works with \?
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Example: egrep with ( )
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% egrep ‘(no)+’ grep-datafile
northwest NW Charles Main 300000.00
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
Print all lines containing one or more consecutive occurrences of the pattern “no”.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Note: grep works with \( \) \+
Example: egrep with (a|b)
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% egrep ‘S(h|u)’ grep-datafile
western WE Sharon Gray 53000.89
southern SO Suan Chin 54500.10
Print all lines containing the uppercase letter “S”, followed by either “h” or “u”.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Note: grep works with \( \) \|
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Example: fgrep
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% fgrep ‘[A-Z]****[0-9]..$5.00’ grep-datafile
Extra [A-Z]****[0-9]..$5.00
Find all lines in the file containing the literal string “[A-Z]****[0-9]..$5.00”. All
characters are treated as themselves. There are no special characters.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Example: Grep with ^
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% grep ‘^n’ grep-datafile
northwest NW Charles Main 300000.00
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
Print all lines beginning with the letter n.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
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Example: grep with $
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% grep ‘\.00$’ grep-datafile
northwest NW Charles Main 300000.00
southeast SE Patricia Hemenway 400000.00
Extra [A-Z]****[0-9]..$5.00
Print all lines ending with a period and exactly two zero numbers.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Example: grep with \char
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% grep ‘5\..’ grep-datafile
Extra [A-Z]****[0-9]..$5.00
Print all lines containing the number 5, followed by a literal period and any
single character.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
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Example: grep with [ ]
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% grep ‘^[we]’ grep-datafile
western WE Sharon Gray 53000.89
eastern EA TB Savage 440500.45
Print all lines beginning with either a “w” or an “e”.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Example: grep with [^]
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% grep ‘\.[^0][^0]$’ grep-datafile
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
eastern EA TB Savage 440500.45
Print all lines ending with a period and exactly two non-zero numbers.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
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Example: grep with x\{m\}
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% grep ‘[0-9]\{6\}\.’ grep-datafile
northwest NW Charles Main 300000.00
southwest SW Lewis Dalsass 290000.73
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Print all lines where there are at least six consecutive numbers followed by a period.
% cat grep-datafile
northwest NW Charles Main 300000.00
western WE Sharon Gray 53000.89
southwest SW Lewis Dalsass 290000.73
southern SO Suan Chin 54500.10
southeast SE Patricia Hemenway 400000.00
eastern EA TB Savage 440500.45
northeast NE AM Main Jr. 57800.10
north NO Ann Stephens 455000.50
central CT KRush 575500.70
Extra [A-Z]****[0-9]..$5.00
Example: grep with \<
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% grep '\