1. Regular Expressions grep
Lecture 6 Lecture 7
Regular Expressions
grep

2. Why Regular Expressions?
Regular expressions are used to describe text patterns/filters
Unix commands/utilities that support regular expressions:
grep(fgrep, egrep) - search a file for a string or regular expression
sed - stream editor
awk (nawk) - pattern scanning and processing language
There are some minor differences between the regular expressions supported by these programs
We will cover the general matching operators first.

3. Character Class [] matches any of the enclosed chars
[abc] matches a single a b or c
[a-z] matches any of abcdef…xyz
[^A-Za-z] matches a single character as long as it is not a letter.
Example: [Dd][Aa][Vv][Ee]
Matches "Dave" or "dave" or "dAVE",
Does not match "ave" or "da"
Exactly one character from the possibilities

4. Regular Expression Operators
Any character (except a metacharacter!) matches itself.
. Matches any single character except newline.
* Matches 0 or more of the immediately preceding R.E.
? Matches 0 or 1 instances of the immediately preceding R.E.
+ Matches 1 or more instances of immediately preceding R.E.
^ Matches the preceding R.E. at the beginning of the line
$ Matches the preceding R.E. at the end of the line
| Matches the R.E. specified before or after this symbol
\ Turn off the special meaning

5. Examples of R.E. x[abc]?x matches "xax" or "xx“
[abc]* matches "aaaaa" or "acbca"
0*10 matches "010" or " "or "10"
^(dog)$ matches lines starting and ending with dog
[\t ]*
(A|a)+b*c?

6. Grouping with parens
If you put a subpattern inside parens you can use + * and ? to the entire subpattern.
a(bc)*d matches "ad" and "abcbcd"
does not match "abcxd" or "bcbcd"

7. Example Christian Scott lives here and will put on a Christmas party
There are around 30 to 35 people invited.
They are:
Tom
Dan
Rhonda Savage
Nicky and Kimberly.
Steve, Suzanne, Ginger and Larry
^[A-Z]..$ ^[A-Z][a-z]*3[0-5] [a-z]*\.
^ *[A-Z][a-z][a-z]$ ^[A-Z][a-z]*[^,][A-Za-z]*$

8. Review: Metacharacters for filename abbreviation
* Matches anything: ls Test*.doc
? Matches any single character
ls Test?.doc
[abc…] Matches any of the enclosed characters: ls T[eE][sS][tT].doc
[a-z] matches any character in a range
ls [a-zA-Z]*
[!abc…] matches any character except those listed: ls [!0-9]*

9. Difference !!
Although there are similarities to the metacharacters used in filename expansion – we are talking about something different!
Filename expansion is done by the shell.
Regular expressions are used by commands (programs).
However, be careful about specifying RE on the command line as a result of this overlap
Good idea to always quote RE with special chars (‘’or “”)on the command line
Example:
% grep ‘[a-z]*’ chap[12]*
Note: filename mask expanded by shell w/o ``

10. grep - search for a string
grep [-bchilnsvw] PATTERN [filename...]
Read files or standard /redirected input
Search for specified pattern in each line
Send results to the standard output
Examples:
%grep ‘^X11’ *- search all files for lines starting with the string “X11”
%grep -v text file - print lines that do not match “text”

11. Regular expressions for grep
c any non special character \c turn off any special meaning of character c ^ beginning of line $ end of line . any single character [...] any of characters in range .… [^....] any single character not in range .… r* zero or more occurrences of r

12. Regular Expressions for grep
\< beginning of word anchor
\<abc matches “abcd” but not “dabc”
\> end of work anchor
abc\> matches “dabc” but not “abcd”
\(…\) stores the pattern …
\(abc\)def matches “abcdef” and stores
abc in \1. So \(abc\)def\1 matches “abcdefabc”. Can store up to 9 matches

13. grep - options Some useful options
-c count number of lines -h do not display filename -l list only the files with matching lines -v display lines that do not match -n print line numbers

14. File db northwest NW Charles Main 3.0 .98 3 34
western WE Sharon Gray
southwest SW Lewis Dalsass
southern SO Suan Chin
southeast SE Patricia Heme
eastern EA TB Savage
northeast NE AM Main Jr
north NO Margot Webber
central CT Ann Stephens

15. grep with pipes Remember, we can use pipes when a file is expected
ls –l | grep ‘\<Feb.*3\>’

16. egrep Extended grep allows for more kinds of regular expressions
unfortunately, egrep regular expressions are not a superset of grep regular expressions
some of grep’s regular expressions are not available in egrep

17. grep vs. egrep new to egrep only in grep
f+ matches one or more occurrences of f
f? matches zero or one occurrences of f
f|g matches f or g
(ab) groups characters a and b together
only in grep
\( … \), \<, \>
Final Note: Different versions of grep/egrep may support different expressions. Make sure to check the man pages.

18. Recommended Reading
Chapter 3
Chapter 4, sections 4.1 – 4.5