CS 497C - Introduction to UNIX
Lecture 28: - Filters Using Regular Expressions - grep and sed
Chin-Chih Chang chang@cs.twsu.edu

grep: Searching for a Pattern
* The sample database shown on Page 434 can be found in 
  http://www.mhhe.com/engcs/compsci/das/data.mhtml
* grep is a filter used to search a file for a pattern.
* It scans a file for the occurrence of a pattern and, depending on the 
  options used, displays: 
 - Lines containing the selected pattern.

grep: Searching for a Pattern
 - Lines not containing the selected pattern. (-v)
 - Line numbers where the pattern occurs. (-n)
 - Number of lines containing the pattern. (-c)
 - Filenames where the pattern occurs. (-l)
* Its syntax treats the first argument as the pattern and the rest as 
  filenames:
grep options pattern filename(s)
$ grep sales emp.lst
* When you use multiword strings as the pattern, you must quote the pattern.

grep Options
$ grep "neil o'bryan" emp.lst
* The -c (count) option counts the occurrences.
$ grep -c directory emp*.lst
* The -n (number) option can be used to display the line numbers containing 
  the pattern.
$ grep -n 'marketing' emp.lst

grep Options
* The -l (list) option displays only the names of files where a pattern has 
  been found.
$ grep -l 'manager' *.lst
* The -i (ignore) option makes the match case-insensitive.
* To look for a pattern that begins with a hyphen, use the -e option.
$ grep -e "-mtime" /var/spool/cron/crontabs/*

grep Options
* In Linux, you can use the -e option to match multiple patterns.
$ grep -e woodhouse -e wood emp.lst
* A regular expression is an ambiguous expression formed with some special 
  and ordinary characters, which is expanded by a command to match more than 
  one string.
* grep uses a regular expression to match a group of similar patterns.

Regular Expressions
* A regular expression uses a character class that encloses a group of 
  characters within a pair of rectangular brackets [].
$ grep "wo[od][de]house" emp.lst
* Regular expressions use the ^(caret) to negate the character class, while 
  the shell use ! (bang).
* A single nonalphabetic string is represented by [^a-zA-Z].

Regular Expressions
* The * (asterisk) matches the zero or more occurrences of the preceding 
  character.
$ grep "wilco[cx]k*s*" emp.lst
* A . (dot) matches a single character. The shell use the ? character to 
  indicate that.
* The dot along with the * (.*) signifies any number of characters, or none.
$ grep "p.*woodhouse" emp.lst

Regular Expressions
* A pattern can be matched at the beginning of a line with a ^, and at the 
  end with a $.
* Because it is the command that interprets these characters, a regular 
  expression should be quoted to prevent the shell from interfering.
$ grep "^2" emp.lst
* The . and * lose their meanings when placed inside the character class. 
  Then you need to escape these characters. 

egrep and fgrep: The Other Members
* egrep extends grep's capabilities. It uses | to delimit multiple patterns.
$ egrep 'woodhouse|woodcock' emp.lst
* fgrep accepts only fixed strings and is faster than the other two.
* Both commands support the -f (file) option to take such patterns from the 
  file.
fgrep -f pat.lst emp.lst