jverkamp.com

Hey, I said that I would follow up on my post about Building Myself a Grep… well here it is!

And I’m actually surprised with myself in how far I actually made it!

You can see the current state of my code on Github. You can install it from that repo (checked out) with cargo install --path .

I mostly worked off the MDN documentation:

Details

Supported Regex Features

Assertions:
- ^ and $ for entire patterns
- Parsing look head/behind (not matched)
Character classes
- Single characters: [abc]
- Ranges: [a-z]
- Negated classes: [^abc]
- Wildcards: .
- Classes: \d/\D for digits, \w/\W for ‘words’, and \s/\S for whitespace
- Escape characters: \t\r\n\v\f
- Control characters: \cX (I’ve never used these)
- Hex and unicode literals: \hXX and \uXXXX
- Disjunction: | (both in capture groups and not)
Groups and back references
- Capture groups: (abc)
- Named capture groups: (?<name>abc)
- Non-capturing groups: (?:abc)
- Flags: (?ims-ims:abc)
  - Both enabling and disabling
  - i and s but not m
- Backreferences: \n
- Named backreferences: \k<name>
Quantifiers
- * for zero or more
- + for one or more
- ? for zero or one
- *?, +?, and ?? for lazy / non-greedy matches
- abc{n} exactly n matches
- abc{n,} at least n matches
- abc{,m} up to m matches
- abc{n,m} at least n and up to m matches (inclusive)
- Lazy matches for all of those

Most of those were fairly straight forward extensions of previous code. In think the most interesting ones were handling the parsing of all the different things that can go in groups (including flags).

For each of them, you can check my git commit history to see how I implemented specific things. It’s mostly one commit per feature, but not always.

Unsupported Regex Features (so far!)

Assertions:
- Word boundaries (\b and \B)
- Look ahead/behind (parsed but not matched)
Character classes
- [\b] for backspace characters
- Long unicode format: \u{XXXXX}
- Unicode properties: \p{...}/\P{...}
Groups and back references:
- m flag / mode: multiline matches

The look ahead/behind is the one I’m most interested in supporting. I don’t even think it will be that hard, I just honestly missed it.

The more interesting one will be the m flag. Currently, I only match lines, so that will be a decently large restructuring. We’ll see.

Supported CLI flags

I’ve made an awful lot of progress on this one too!

$ jp-grep --help

A custom grep implementation; always behaves as egrep

Usage: jp-grep [OPTIONS] [PATTERN] [PATHS]...

Arguments:
  [PATTERN]   The regular expression to evaluate; may also be specified with -e
  [PATHS]...  Paths to search for matches; if none are provided read from stdin

Options:
  -A, --after-context <AFTER_CONTEXT>
          Lines of context to print after each match
  -B, --before-context <BEFORE_CONTEXT>
          Lines of context to print before each match
  -C, --context <CONTEXT>
          Lines to print both before and after
  -c, --count
          Only print the matching count
  -E, --extended-regexp
          Extended regex mode (egrep); this option is ignored (always true)
  -e, --regexp <ADDITIONAL_PATTERNS>
          Additional patterns, will return a line if any match
  -h, --no-filename
          Never print filenames
      --help
          Display this help message
  -i, --ignore-case
          Default to case insensitive match
  -n, --line-number
          Print line numbers before matches and context
  -r, --recursive
          Recursively add any directories (-R also works)
  -v, --invert-match
          Invert the match; only print lines that don't match any pattern
  -V, --version
          Print version

Of those, the context flags (-A, -B, and -C) were probably the most tricky, since I basically had to implement a circular buffer for them. I could have just read the entire file into memory, but from the beginning, I didn’t want to do that.

-E is a little silly, since that’s the only grep pattern I support (and the only one I actually use in grep, so that’s fair).

So far as supporting multiple files, recursive search, and stdin, read the section on collecting files later.

So far as printing (handling line numbers and file names), read the section on printing lines.

Overall, pretty fun code.

Unsupported CLI flags

So far, there are a bunch of flags that I don’t support for grep. Of those, there are a bunch that I don’t intend to support (like built in compression support and properly dealing with symlinks).

The things that I would still like to support though are:

Input options:
- -f file/--file=file - Read patterns from file
Output options:
- -a/--text - Currently I always have this set; I don’t treat binary files differently
- -L/--files-without-match - only print files that don’t match
- -o/--only-matching - only print the matching groups; I have the groups for backreferences, use them!
File filtering - files to include/exclude (useful with recursive matches):
- --exclude pattern
- --exclude-dir pattern
- --include pattern
- --include-dir pattern

That’s not too bad, all things consider.

Error handling

One thing that I actually played a bit with this time around was custom error handling in the parser. Rather than just returning &str all over the place for Err types, I made my own:

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum ParserError {
    RemainingInput,
    UnexpectedEnd,
    InvalidCharacter(char, &'static str),
    InvalidUnicodeCodePoint(u32),
    InvalidRange(char, char),
    InvalidRepeatRange(u32, u32),
}

impl std::fmt::Display for ParserError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ParserError::RemainingInput => write!(f, "Unexpected input after parsing"),
            ParserError::UnexpectedEnd => write!(f, "Unexpected end of input"),
            ParserError::InvalidCharacter(c, expected) => {
                write!(f, "Invalid character '{}', expected {}", c, expected)
            }
            ParserError::InvalidUnicodeCodePoint(code_point) => {
                write!(f, "Invalid unicode code point: {}", code_point)
            }
            ParserError::InvalidRange(start, end) => {
                write!(f, "Invalid range: {}-{}", start, end)
            }
            ParserError::InvalidRepeatRange(start, end) => {
                write!(f, "Invalid range: {}-{}", start, end)
            }
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct ParserErrorWithPosition {
    pub position: usize,
    pub error: ParserError,
}

The WithPosition type also lets me pinpoint exactly where in a pattern I failed:

jp-grep 'this is some long complicated pattern, \hXX see?'

Error parsing regex: Invalid character 'X', expected hex digit
| this is some long complicated pattern, \hXX see?
|                                          ^

That’s pretty neat and I hope helpful! 😄

Expanding past CodeCrafters

Overall, I’m pretty happy with this project. It’s got a pretty decent chunk of code, including…

$ jp-grep -c -v -e '//' -e '^\s*$' **/*.rs

1241

…over 1000 lines of Rust code, including tests but not blank lines or comments. 😄

I’ll probably pick this up at least once more.

Now… will I actually use this? Probably not. But it was certainly interesting to write.

Other than that, was CodeCrafters actually helpful for this? Middling. It was the kick I needed to actually do it (I’ve been meaning to write this for years at this point) and once I was started, I could finish it. On the other hand, the output format they require was a bit annoying at times, I’ve mostly moved away from that.

Still, worth I think. I’ll probably continue to do their free programs. Kafka is up next. Whee servers!

JP's Blog

Finish Myself a Grep