Project 2
CS221: C and Systems Programming – Fall 2019
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.
These are the only special characters you have to handle. With the exception of the null char that terminates a string, you should not have to handle any other character in any special way. You may assume that your code will not be run against patterns that don’t make sense. You must follow the specification strictly - so you should neither include any library other than those specified in the skeleton, nor copy and paste code from other libraries. You may not use any code you find online
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