COS 126 Lecture 14: Pattern Matching

NOTE: See also Notes on Formal Language

grep

Most common use is for finding a particular word or short string in a file. This is analogous to searching for a Web document via Yahoo. The syntax is:

grep expression filename(s)

For example, you might want to find all the lines of code in your C files which refer to a particular variable.

grep 'count' *.c

How can you learn about grep?

1. TRY IT!!

2. there is a link to documentation with many examples accessible via the 126 FAQ list

3. type (on a Unix machine):

man grep

You can copy the manual page for grep to a file using '>' and then print that file. You should learn some of the main options for grep.

Here's some hints and highlights about grep:

• use single quotes around the expression
• [ ]
  • Meaning: one of (in range).
  • Examples:

[a3][c5] => a or 3 followed by c or 5, so valid matches are ac,a5,3c,35
[a-z] => any lowercase character between a and z

• ^
  • Meaning: 'not' if it's inside brackets. Otherwise, it indicates that whatever it precedes must be the first thing on a matching line.
  • Examples:

[^ a b c] => matches any lines in which at least one character is not 
a, b, or c
'^a' => matches any lines which start with an 'a'

• . (a period)
  • Meaning: any single character (like a wildcard)
  • Examples:

'^c.b' => matches lines that start with a c and have b as the third character
'.' => matches any lines that have any characters

• *
  • Meaning: zero or more of what precedes it
  • Examples:

'^c.*b' => matches lines that start with c and have at least one b
'c.*b' => matches any lines that have a c followed by a b (not necessarily adjacent)

• +
  • Meaning: one or more of what precedes it
  • Examples:

'c+b' => matches lines with cb, ccb, cccb, ccccb, etc. as substrings
'c.+b' => matches lines with cab, caab, cadb, etc. but not cb

• ?
  • Meaning: zero or one of what precedes it
  • Examples:

'c.?b' => matches cb and cab but not caab (and other strings...)

• $
  • Meaning: whatever precedes it must be the last thing on a matching line
  • Examples:

'c.*b$' => matches any lines that have a c and end with a b
'^c.*b$' => matches any lines that start with c and end with b

• |
  • Meaning: or (in egrep or grep -E)
  • Example:

x | y => matches lines with x or y

• ()
  • Meaning: parentheses are used for grouping
    
• /
  • Meaning: the backslash character is used for escaping characters with reserved meaning (like a period)
    
• {some number}
  • Meaning: repeat the preceding expression a given number of times
  • Examples:

c{3} => matches lines with 3 consecutive c's
(bc){2} => matches lines with the pattern bcbc

• grep -i expression file
  • by default, grep is case sensitive. The -i flag makes the match case insensitive
• grep -c expression file
  • return only the number of matching lines, instead of printing them
• grep -v expression file
  • return all lines in the file not matching the expression

awk

Try it out for yourself or read about it in a UNIX reference guide.