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Project 2
CS221: C and Systems Programming – Fall 2018
Deadline: November 12, 2018 at 11:59pm
Restricted grep (rgrep)
grep is a UNIX utility that is used to search for patterns in text files. It’s a powerful and versatile tool, and in
this project you will implement a version that, while simplified, should still be useful.1
Your project is to complete the implementation of rgrep, our simplified, restricted grep.rgrep is “restricted” in
the sense that the patterns it matches only support a few regular operators (the easier ones). The way rgrep is
used is that a pattern is specified on the command line. rgrep then reads lines from its standard input and prints
them out on its standard output if and only if the pattern “matches” the line. For example, we can use rgrep to
search for lines that contain text file names that are at least 3 characters long (plus the extension) in a file like the
following:
#so you can see what lines are in the file:
$ cat testin
a.out
cs221.txt
cs221.pdf
usf.txt
nope.pdf
.txt
$ ./rgrep ‘.\.txt’ < testin
cs221.txt
usf.txt
What’s going on here? rgrep was given the pattern ".\.txt"; it printed only the lines from its standard input
that matched this pattern. How can you tell if a line matches the pattern? A line matches a pattern if the pattern
“appears” somewhere inside the line. In the absence of any special operators, seeing if a line matches a pattern
reduces to seeing if the pattern occurs as a substring anywhere in the line. So for most characters, their meaning
in a pattern is just to match themselves in the target string. However, there are a few special clauses you must
implement:
.(period) Matches any character
+(plus sign) The preceding character may appear 1 or more times (in other words, the preceding character
can be repeated several times in a row).
?(question
mark)
The preceding character may appear between 0 and 1 times (in other words, the preceding
character is optional).
\(backslash) “Escapes” the following character, nullifying any special meaning it has.
So, here are some examples of patterns and the kinds of lines they match.
( An open parenthesis must appear somewhere in the line.
hey+ Matches a line that contains the string “hey” followed by any number (0 or more) of y’s.
str?ing Matches lines that contain the substrings “string” or “sting”, since the “r” is optional..
z.z\.txt Matches lines that contain the substring “zaz.txt”, “zbz.txt”, etc., where the character between the
z’s can be anything, including a period.
1Type man grep in terminal for more detailed information on how grep works.
