GoGrep

This repo contains the program I wrote for the "Build Your Own grep" Challenge on codecrafters.io. I've been really enjoying both CodeCrafters and writing Go, so as yet another month has come to pass and they're again offering a free challenge, how can I say no? (here's a link to last month's)

And yet.

"Make your own grep," they said; "Regex is fun," they said. All I have to say is that I'm thankful CodeCrafters kept it nice and simple for what tests they'd run. I hate to let down Rob and Brian, but summer is almost over, and I'd like to get off Regex's wild ride.

— Jason, August 2024

From the CodeCrafters overview:

Regular expressions (Regexes, for short) are patterns used to match character combinations in strings. grep is a CLI tool for searching using Regexes.

In this challenge you'll build your own implementation of grep. Along the way we'll learn about Regex syntax, how parsers/lexers work, and how regular expressions are evaluated.

Update: Sept 2, 2024

Sure, to finish the challenge we just had to implement some RegEx and say when there wasn't a match with stdin. But now it actually works like grep! The program can read in a file and write matching lines to stdout. I haven't updated test.sh with relevant test cases but feel free to try it yourself:

• Make a test file:

  touch testing
  echo cats and dogs >> testing
  echo cats and dogo >> testing
  echo dogs and cats >> testing
  echo dogs and cata >> testing
  

• Run the program:

  echo testing:
  cat testing
  echo "\nmatches:"
  go run src/main.go -E '[tg](a|s) and \w\w[tg]\1' testing
  rm testing
  

• You should see only 2 of the 4 lines printed as matches. (The ones with correct spellings for "dogs" and "cats" since the backreference \1 will fail when neither the o nor a matches the previously matched s for that line.)

Usage

• Run the program using go run:

  go run src/main.go
  

• Input is taken from stdin:

  echo caaaats | go run src/main.go
  

• Specify a pattern to match against with the -E option:

  echo caaaats | go run src/main.go -E ca+t
  

• You can view very ugly debug information with the --debug flag:

  echo caaaats | go run src/main.go -E ca+t --debug
  

Features

• Matches literal characters
• Matches digits with \d
• Matches alphanumeric characters with \w
• Maches one of a set of characters with [abc]
• Negatively maches one of a set of characters with [^abc]
• Matches the start of a string with ^
• Matches the end of a string with $
• Matches one or more times with +
• Matches zero or one times with ?
• Supports alternation with | like in (cat|dog)
• Supports backreferences with \1 like in super(man|woman) and bat\1
• Is otherwise very limited and buggy (probably)

Automated Tests

While working through the challenge stages, I maintained a test script to help verify new additions and catch regressions before submitting:

./test.sh