Handle grep output769-git-grep-rebase

- Handle standard filepath:code and filepath:line_number:code output
  as produced by `git grep`, `rg -H`, `grep -H`, etc (with -n for line
  numbers).

- Retain the match highlighting as produced by the grep tool, and
  expose it in delta's color output styled with grep-match-style.
  (Note that --color=always is needed to retain the color if piping
  into delta, but not for `git grep` when delta is configured as git's
  pager)

- Special handling of -p, and -W options of `git grep`: these display
  the function context in which the matches occur.

- `navigate` keybindings jump between match function contexts under
  `git grep -p` and between matching lines under `git grep -W`.

Thanks @zachriggle for the proposal.
Fixes #769

13 files changed, 1041 insertions, 5 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 0c081ac2..af34912c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -356,6 +356,8 @@ dependencies = [
  "lazy_static",
  "pathdiff",
  "regex",
+ "serde",
+ "serde_json",
  "shell-words",
  "smol_str",
  "structopt",
@@ -806,6 +808,9 @@ name = "serde"
 version = "1.0.118"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "06c64263859d87aa2eb554587e2d23183398d617427327cf2b3d0ed8c69e4800"
+dependencies = [
+ "serde_derive",
+]
 
 [[package]]
 name = "serde_derive"
@@ -820,9 +825,9 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.61"
+version = "1.0.70"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4fceb2595057b6891a4ee808f70054bd2d12f0e97f1cbb78689b59f676df325a"
+checksum = "e277c495ac6cd1a01a58d0a0c574568b4d1ddf14f59965c6a58b8d96400b54f3"
 dependencies = [
  "itoa",
  "ryu",
diff --git a/Cargo.toml b/Cargo.toml
index e7181ddc..5aeeb8ea 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -31,6 +31,8 @@ itertools = "0.10.1"
 lazy_static = "1.4"
 pathdiff = "0.2.1"
 regex = "1.4.6"
+serde = { version = "1.0.118", features = ["derive"] }
+serde_json = "1.0.70"
 shell-words = "1.0.0"
 smol_str = "0.1.18"
 structopt = "0.3.25"
diff --git a/README.md b/README.md
index 8cc6b0a0..8a2aa1f4 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@
     show = delta
     log = delta
     blame = delta
+    grep = delta
     reflog = delta
 
 [interactive]
@@ -64,6 +65,7 @@ Code evolves, and we all spend time studying diffs. Delta aims to make this both
   - [Choosing colors (styles)](#choosing-colors-styles)
   - [Line numbers](#line-numbers)
   - [Side-by-side view](#side-by-side-view)
+  - [Grep](#grep)
   - ["Features": named groups of settings](#features-named-groups-of-settings)
   - [Custom themes](#custom-themes)
   - [diff-highlight and diff-so-fancy emulation](#diff-highlight-and-diff-so-fancy-emulation)
@@ -151,6 +153,7 @@ Here's what `git show` can look like with git configured to use delta:
 - Git style strings (foreground color, background color, font attributes) are supported for >20 stylable elements
 - Side-by-side view with line-wrapping
 - Line numbering
+- Handles grep output with file paths from `rg`, `git grep`, `grep`, etc
 - `diff-highlight` and `diff-so-fancy` emulation modes
 - Stylable box/line decorations to draw attention to commit, file and hunk header sections.
 - Support for Git's `--color-moved` feature.
@@ -410,6 +413,15 @@ In contrast, the long replacement line in the right panel overflows by almost an
 For control over the details of line wrapping, see `--wrap-max-lines`, `--wrap-left-symbol`, `--wrap-right-symbol`, `--wrap-right-percent`, `--wrap-right-prefix-symbol`, `--inline-hint-style`.
 Line wrapping was implemented by @th1000s.
 
+### Grep
+
+Delta applies syntax-highlighting and other enhancements to standard grep output such as from `git grep`, [ripgrep](https://github.com/BurntSushi/ripgrep/) (aka `rg`), grep, etc.
+To use with `git grep`, set delta as the pager for `grep` in the `[pager]` section of your gitconfig. See the example at the [top of the page](#get-started).
+Output from other grep tools can be piped to delta: e.g. `rg -Hn --color=always`, `grep -Hn --color=always`, etc.
+To customize the colors and syntax highlighting, see `grep-match-line-style`, `grep-match-word-style`, `grep-contexct-line-style`, `grep-file-style`, `grep-line-number-style`.
+Ripgrep's `rg --json` output format is supported; this avoids certain file name parsing ambiguities that are inevitable with the standard grep output formats.
+Note that `git grep` can display the "function context" for matches and that delta handles this output specially: see the `-p` and `-W` options of `git grep`.
+
 ### "Features": named groups of settings
 
 All delta options can go under the `[delta]` section in your git config file. However, you can also use named "features" to keep things organized: these are sections in git config like `[delta "my-feature"]`. Here's an example using two custom features:
diff --git a/src/cli.rs b/src/cli.rs
index aa40ce1e..1d0c7640 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -441,6 +441,40 @@ pub struct Opt {
     )]
     pub blame_timestamp_format: String,
 
+    #[structopt(long = "grep-match-line-style")]
+    /// Style (foreground, background, attributes) for matching lines of code in
+    /// grep output. See STYLES section. Defaults to plus-style.
+    pub grep_match_line_style: Option<String>,
+
+    #[structopt(long = "grep-match-word-style")]
+    /// Style (foreground, background, attributes) for the specific matching
+    /// substrings within a matching line of code in grep output. See STYLES
+    /// section. Defaults to plus-style.
+    pub grep_match_word_style: Option<String>,
+
+    #[structopt(long = "grep-context-line-style")]
+    /// Style (foreground, background, attributes) for non-matching lines of
+    /// code in grep output. See STYLES section. Defaults to zero-style.
+    pub grep_context_line_style: Option<String>,
+
+    #[structopt(long = "grep-file-style")]
+    /// Style (foreground, background, attributes) for file paths in grep
+    /// output. See STYLES section. Defaults to hunk-header-file-path-style.
+    pub grep_file_style: Option<String>,
+
+    #[structopt(long = "grep-line-number-style")]
+    /// Style (foreground, background, attributes) for line numbers in grep
+    /// output. See STYLES section. Defaults to hunk-header-line-number-style.
+    pub grep_line_number_style: Option<String>,
+
+    #[structopt(long = "grep-separator-symbol", default_value = ":")]
+    /// Symbol used in grep output to separate file path (and line number) from
+    /// the line of file contents. Defaults to ":" for both match and context
+    /// lines, since many terminal emulators recognize constructs like
+    /// "/path/to/file:7:". However, standard grep output uses "-" for context
+    /// lines: set this option to "keep" to keep the original separator symbols.
+    pub grep_separator_symbol: String,
+
     /// Default language used for syntax highlighting when this cannot be
     /// inferred from a filename. It will typically make sense to set this in
     /// per-repository git config (.git/config)
diff --git a/src/config.rs b/src/config.rs
index 4ed7f27a..2b476f9e 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -79,6 +79,12 @@ pub struct Config {
     pub git_config: Option<GitConfig>,
     pub git_minus_style: Style,
     pub git_plus_style: Style,
+    pub grep_context_line_style: Style,
+    pub grep_file_style: Style,
+    pub grep_line_number_style: Style,
+    pub grep_match_line_style: Style,
+    pub grep_match_word_style: Style,
+    pub grep_separator_symbol: String,
     pub hunk_header_file_style: Style,
     pub hunk_header_line_number_style: Style,
     pub hunk_header_style_include_file_path: bool,
@@ -217,6 +223,32 @@ impl From<cli::Opt> for Config {
             _ => *style::GIT_DEFAULT_PLUS_STYLE,
         };
 
+        let grep_match_line_style = if let Some(s) = opt.grep_match_line_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            zero_style
+        };
+        let grep_match_word_style = if let Some(s) = opt.grep_match_word_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            plus_emph_style
+        };
+        let grep_context_line_style = if let Some(s) = opt.grep_context_line_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            zero_style
+        };
+        let grep_file_style = if let Some(s) = opt.grep_file_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_file_style
+        };
+        let grep_line_number_style = if let Some(s) = opt.grep_line_number_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_line_number_style
+        };
+
         let blame_palette = make_blame_palette(opt.blame_palette, opt.computed.is_light_mode);
 
         let file_added_label = opt.file_added_label;
@@ -285,6 +317,12 @@ impl From<cli::Opt> for Config {
             file_style,
             git_config: opt.git_config,
             git_config_entries: opt.git_config_entries,
+            grep_context_line_style,
+            grep_file_style,
+            grep_line_number_style,
+            grep_match_line_style,
+            grep_match_word_style,
+            grep_separator_symbol: opt.grep_separator_symbol,
             hunk_header_file_style,
             hunk_header_line_number_style,
             hunk_header_style,
diff --git a/src/delta.rs b/src/delta.rs
index fbf45c53..5f439139 100644
--- a/src/delta.rs
+++ b/src/delta.rs
@@ -23,6 +23,7 @@ pub enum State {
     SubmoduleLog, // In a submodule section, with gitconfig diff.submodule = log
     SubmoduleShort(String), // In a submodule section, with gitconfig diff.submodule = short
     Blame(String, Option<String>), // In a line of `git blame` output (commit, repeat_blame_line).
+    Grep(String, Option<String>), // In a line of `git grep` output (file, repeat_grep_line).
     Unknown,
     // The following elements are created when a line is wrapped to display it:
     HunkZeroWrapped,  // Wrapped unchanged line
@@ -121,6 +122,7 @@ impl<'a> StateMachine<'a> {
                 || self.handle_submodule_short_line()?
                 || self.handle_hunk_line()?
                 || self.handle_blame_line()?
+                || self.handle_grep_line()?
                 || self.should_skip_line()
                 || self.emit_line_unchanged()?;
         }
@@ -133,7 +135,13 @@ impl<'a> StateMachine<'a> {
     fn ingest_line(&mut self, raw_line_bytes: &[u8]) {
         // TODO: retain raw_line as Cow
         self.raw_line = String::from_utf8_lossy(raw_line_bytes).to_string();
-        if self.config.max_line_length > 0 && self.raw_line.len() > self.config.max_line_length {
+        if self.config.max_line_length > 0
+            && self.raw_line.len() > self.config.max_line_length
+            // We must not truncate ripgrep --json output
+            // TODO: An alternative might be to truncate `line` but retain
+            // `raw_line` untruncated?
+            && !self.raw_line.starts_with('{')
+        {
             self.raw_line = ansi::truncate_str(
                 &self.raw_line,
                 self.config.max_line_length,
diff --git a/src/handlers/file_meta.rs b/src/handlers/file_meta.rs
index 8a1ea0d3..bea05625 100644
--- a/src/handlers/file_meta.rs
+++ b/src/handlers/file_meta.rs
@@ -210,7 +210,7 @@ fn get_file_extension_from_file_meta_line_file_path(path: &str) -> Option<&str>
 }
 
 /// Attempt to parse input as a file path and return extension as a &str.
-fn get_extension(s: &str) -> Option<&str> {
+pub fn get_extension(s: &str) -> Option<&str> {
     let path = Path::new(s);
     path.extension()
         .and_then(|e| e.to_str())
diff --git a/src/handlers/grep.rs b/src/handlers/grep.rs
new file mode 100644
index 00000000..044c7400
--- /dev/null
+++ b/src/handlers/grep.rs
@@ -0,0 +1,711 @@
+// TODO
+// Bad parsing: "etc/examples/119-within-line-edits:4:repo=$(mktemp -d)"
+// Parsing "Makefile"
+// Inspect process tree once
+use std::borrow::Cow;
+
+use lazy_static::lazy_static;
+use regex::Regex;
+use serde::Deserialize;
+use unicode_segmentation::UnicodeSegmentation;
+
+use crate::ansi;
+use crate::delta::{State, StateMachine};
+use crate::handlers::{self, ripgrep_json};
+use crate::paint::{self, BgShouldFill, StyleSectionSpecifier};
+use crate::style::Style;
+use crate::utils;
+
+#[derive(Debug, PartialEq)]
+pub struct GrepLine<'b> {
+    pub path: Cow<'b, str>,
+    pub line_number: Option<usize>,
+    pub line_type: LineType,
+    pub code: Cow<'b, str>,
+    pub submatches: Option<Vec<(usize, usize)>>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum LineType {
+    ContextHeader,
+    Context,
+    Match,
+    Ignore,
+}
+
+struct GrepOutputConfig {
+    add_navigate_marker_to_matches: bool,
+    render_context_header_as_hunk_header: bool,
+    pad_line_number: bool,
+}
+
+lazy_static! {
+    static ref OUTPUT_CONFIG: GrepOutputConfig = make_output_config();
+}
+
+impl<'a> StateMachine<'a> {
+    /// If this is a line of git grep output then render it accordingly. If this
+    /// is the first grep line, then set the syntax-highlighter language.
+    pub fn handle_grep_line(&mut self) -> std::io::Result<bool> {
+        self.painter.emit()?;
+        let mut handled_line = false;
+
+        // TODO: It should be possible to eliminate some of the .clone()s and
+        // .to_owned()s.
+        let (_previous_file, repeat_grep_line, try_parse) = match &self.state {
+            State::Grep(file, repeat_grep_line) => {
+                (Some(file.as_str()), repeat_grep_line.clone(), true)
+            }
+            State::Unknown => (None, None, true),
+            _ => (None, None, false),
+        };
+        if try_parse {
+            if let Some(mut grep_line) = parse_grep_line(&self.line) {
+                if matches!(grep_line.line_type, LineType::Ignore) {
+                    handled_line = true;
+                    return Ok(handled_line);
+                }
+
+                // Emit syntax-highlighted code
+                // TODO: Determine the language less frequently, e.g. only when the file changes.
+                if let Some(lang) = handlers::file_meta::get_extension(&grep_line.path)
+                    .or_else(|| self.config.default_language.as_deref())
+                {
+                    self.painter.set_syntax(Some(lang));
+                    self.painter.set_highlighter();
+                }
+                self.state = State::Grep(grep_line.path.to_string(), repeat_grep_line);
+
+                match (
+                    &grep_line.line_type,
+                    OUTPUT_CONFIG.render_context_header_as_hunk_header,
+                ) {
+                    // Emit context header line
+                    (LineType::ContextHeader, true) => handlers::hunk_header::write_hunk_header(
+                        &grep_line.code,
+                        &[(grep_line.line_number.unwrap_or(0), 0)],
+                        &mut self.painter,
+                        &self.line,
+                        &grep_line.path,
+                        self.config,
+                    )?,
+                    _ => {
+                        if self.config.navigate {
+                            write!(
+                                self.painter.writer,
+                                "{}",
+                                match (
+                                    &grep_line.line_type,
+                                    OUTPUT_CONFIG.add_navigate_marker_to_matches
+                                ) {
+                                    (LineType::Match, true) => "• ",
+                                    (_, true) => "  ",
+                                    _ => "",
+                                }
+                            )?
+                        }
+                        // Emit file & line-number
+                        let separator = if self.config.grep_separator_symbol == "keep" {
+                            // grep, rg, and git grep use ":" for matching lines
+                            // and "-" for non-matching lines (and `git grep -W`
+                            // uses "=" for a context header line).
+                            match grep_line.line_type {
+                                LineType::Match => ":",
+                                LineType::Context => "-",
+                                LineType::ContextHeader => "=",
+                                LineType::Ignore => "",
+                            }
+                        } else {
+                            // But ":" results in a "file/path:number:"
+                            // construct that terminal emulators are more likely
+                            // to recognize and render as a clickable link. If
+                            // navigate is enabled then there is already a good
+                            // visual indicator of match lines (in addition to
+                            // the grep-match-style highlighting) and so we use
+                            // ":" for matches and non-matches alike.
+                            &self.config.grep_separator_symbol
+                        };
+                        write!(
+                            self.painter.writer,
+                            "{}",
+                            paint::paint_file_path_with_line_number(
+                                grep_line.line_number,
+                                &grep_line.path,
+                                OUTPUT_CONFIG.pad_line_number,
+                                separator,
+                                true,
+                                Some(self.config.grep_file_style),
+                                Some(self.config.grep_line_number_style),
+                                self.config
+                            )
+                        )?;
+
+                        // Emit code line
+                        let code_style_sections =
+                            match (&grep_line.line_type, &grep_line.submatches) {
+                                (LineType::Match, Some(submatches)) => {
+                                    // We expand tabs at this late stage because
+                                    // the tabs are escaped in the JSON, so
+                                    // expansion must come after JSON parsing.
+                                    // (At the time of writing, we are in this
+                                    // arm iff we are handling `ripgrep --json`
+                                    // output.)
+                                    grep_line.code = self
+                                        .painter
+                                        .expand_tabs(grep_line.code.graphemes(true))
+                                        .into();
+                                    make_style_sections(
+                                        &grep_line.code,
+                                        submatches,
+                                        self.config.grep_match_word_style,
+                                        self.config.grep_match_line_style,
+                                    )
+                                }
+                                (LineType::Match, None) => {
+                                    // HACK: We need tabs expanded, and we need
+                                    // the &str passed to
+                                    // `get_code_style_sections` to live long
+                                    // enough. But at the point it is guaranteed
+                                    // that this handler is going to handle this
+                                    // line, so mutating it is acceptable.
+                                    self.raw_line =
+                                        self.painter.expand_tabs(self.raw_line.graphemes(true));
+                                    get_code_style_sections(
+                                        &self.raw_line,
+                                        self.config.grep_match_word_style,
+                                        self.config.grep_match_line_style,
+                                        &grep_line,
+                                    )
+                                    .unwrap_or(
+                                        StyleSectionSpecifier::Style(
+                                            self.config.grep_match_line_style,
+                                        ),
+                                    )
+                                }
+                                _ => StyleSectionSpecifier::Style(
+                                    self.config.grep_context_line_style,
+                                ),
+                            };
+                        self.painter.syntax_highlight_and_paint_line(
+                            &format!("{}\n", grep_line.code),
+                            code_style_sections,
+                            self.state.clone(),
+                            BgShouldFill::default(),
+                        )
+                    }
+                }
+                handled_line = true
+            }
+        }
+        Ok(handled_line)
+    }
+}
+
+fn make_style_sections<'a>(
+    line: &'a str,
+    submatches: &[(usize, usize)],
+    match_style: Style,
+    non_match_style: Style,
+) -> StyleSectionSpecifier<'a> {
+    let mut sections = Vec::new();
+    let mut curr = 0;
+    for (start_, end_) in submatches {
+        let (start, end) = (*start_, *end_);
+        if start > curr {
+            sections.push((non_match_style, &line[curr..start]))
+        };
+        sections.push((match_style, &line[start..end]));
+        curr = end;
+    }
+    if curr < line.len() {
+        sections.push((non_match_style, &line[curr..]))
+    }
+    StyleSectionSpecifier::StyleSections(sections)
+}
+
+// Return style sections describing colors received from git.
+fn get_code_style_sections<'b>(
+    raw_line: &'b str,
+    match_style: Style,
+    non_match_style: Style,
+    grep: &GrepLine,
+) -> Option<StyleSectionSpecifier<'b>> {
+    if let Some(raw_code_start) = ansi::ansi_preserving_index(
+        raw_line,
+        match grep.line_number {
+            Some(n) => format!("{}:{}:", grep.path, n).len(),
+            None => grep.path.len() + 1,
+        },
+    ) {
+        let match_style_sections = ansi::parse_style_sections(&raw_line[raw_code_start..])
+            .iter()
+            .map(|(ansi_term_style, s)| {
+                if ansi_term_style.foreground.is_some() {
+                    (match_style, *s)
+                } else {
+                    (non_match_style, *s)
+                }
+            })
+            .collect();
+        Some(StyleSectionSpecifier::StyleSections(match_style_sections))
+    } else {
+        None
+    }
+}
+
+fn make_output_config() -> GrepOutputConfig {
+    match utils::process::git_grep_command_options() {
+        Some((longs, shorts)) if shorts.contains("-W") || longs.contains("--function-context") => {
+            // --function-context is in effect: i.e. the entire function is
+            // being displayed. In that case we don't render the first line as a
+            // header, since the second line is the true next line, and it will
+            // be more readable to have these displayed normally. We do add the
+            // navigate marker, since match lines will be surrounded by (many)
+            // non-match lines. And, since we are printing (many) successive lines
+            // of code, we pad line numbers <100 in order to maintain code
+            // alignment up to line 9999.
+            GrepOutputConfig {
+                render_context_header_as_hunk_header: false,
+                add_navigate_marker_to_matches: true,
+                pad_line_number: true,
+            }
+        }
+        Some((longs, shorts)) if shorts.contains("-p") || longs.contains("--show-function") => {
+            // --show-function is in effect, i.e. the function header is being
+            // displayed, along with matches within the function. Therefore we
+            // render the first line as a header, but we do not add the navigate
+            // marker, since all non-header lines are matches.
+            GrepOutputConfig {
+                render_context_header_as_hunk_header: true,
+                add_navigate_marker_to_matches: false,
+                pad_line_number: false,
+            }
+        }
+        _ => GrepOutputConfig {
+            render_context_header_as_hunk_header: true,
+            add_navigate_marker_to_matches: false,
+            pad_line_number: false,
+        },
+    }
+}
+
+enum GrepLineRegex {
+    FilePathWithFileExtension,
+    FilePathWithoutSeparatorCharacters,
+}
+
+lazy_static! {
+    static ref GREP_LINE_REGEX_ASSUMING_FILE_EXTENSION: Regex =
+        make_grep_line_regex(GrepLineRegex::FilePathWithFileExtension);
+}
+
+lazy_static! {
+    static ref GREP_LINE_REGEX_ASSUMING_NO_INTERNAL_SEPARATOR_CHARS: Regex =
+        make_grep_line_regex(GrepLineRegex::FilePathWithoutSeparatorCharacters);
+}
+
+// See tests for example grep lines
+fn make_grep_line_regex(regex_variant: GrepLineRegex) -> Regex {
+    // Grep tools such as `git grep` and `rg` emit lines like the following,
+    // where "xxx" represents arbitrary code. Note that there are 3 possible
+    // "separator characters": ':', '-', '='.
+
+    // The format is ambiguous, but we attempt to parse it.
+
+    // src/co-7-fig.rs:xxx
+    // src/co-7-fig.rs:7:xxx
+    // src/co-7-fig.rs-xxx
+    // src/co-7-fig.rs-7-xxx
+    // src/co-7-fig.rs=xxx
+    // src/co-7-fig.rs=7=xxx
+
+    // Makefile:xxx
+    // Makefile:7:xxx
+    // Makefile-xxx
+    // Makefile-7-xxx
+
+    // Make-7-file:xxx
+    // Make-7-file:7:xxx
+    // Make-7-file-xxx
+    // Make-7-file-7-xxx
+
+    let file_path = match regex_variant {
+        GrepLineRegex::FilePathWithFileExtension => {
+            r"
+        (                        # 1. file name (colons not allowed)
+            [^:\ ]                  # a file name cannot start with whitespace
+            [^:]*                   # anything
+            \.[^.\ :=-]{1,6}        # extension
+        )    
+        "
+        }
+        GrepLineRegex::FilePathWithoutSeparatorCharacters => {
+            r"
+        (                        # 1. file name (colons not allowed)
+            [^:\ =-]                # a file name cannot start with whitespace
+            [^:=-]*                 # anything except separators
+            [^:\ ]                  # a file name cannot end with whitespace
+        )    
+        "
+        }
+    };
+
+    Regex::new(&format!(
+        "(?x)
+^
+{file_path}
+(?:
+    (                    
+        :                # 2. match marker
+        (?:([0-9]+):)?   # 3. optional: line number followed by second match marker
+    )
+    |
+    (
+        -                # 4. nomatch marker
+        (?:([0-9]+)-)?   # 5. optional: line number followed by second nomatch marker
+    )
+    |
+    (
+        =                # 6. match marker
+        (?:([0-9]+)=)?   # 7. optional: line number followed by second header marker
+    )
+)
+(.*)                     # 8. code (i.e. line contents)
+$
+",
+        file_path = file_path
+    ))
+    .unwrap()
+}
+
+pub fn parse_grep_line(line: &str) -> Option<GrepLine> {
+    if line.starts_with('{') {
+        return ripgrep_json::parse_line(line);
+    }
+    [
+        &*GREP_LINE_REGEX_ASSUMING_FILE_EXTENSION,
+        &*GREP_LINE_REGEX_ASSUMING_NO_INTERNAL_SEPARATOR_CHARS,
+    ]
+    .iter()
+    .find_map(|regex| _parse_grep_line(*regex, line))
+}
+
+pub fn _parse_grep_line<'b>(regex: &Regex, line: &'b str) -> Option<GrepLine<'b>> {
+    let caps = regex.captures(line)?;
+    let file = caps.get(1).unwrap().as_str().into();
+    let (line_type, line_number) = &[
+        (2, LineType::Match),
+        (4, LineType::Context),
+        (6, LineType::ContextHeader),
+    ]
+    .iter()
+    .find_map(|(i, line_type)| {
+        if caps.get(*i).is_some() {
+            let line_number: Option<usize> =
+                caps.get(i + 1).map(|m| m.as_str().parse().ok()).flatten();
+            Some((*line_type, line_number))
+        } else {
+            None
+        }
+    })
+    .unwrap(); // The regex matches so one of the three alternatrives must have matched
+    let code = caps.get(8).unwrap().as_str().into();
+
+    Some(GrepLine {
+        path: file,
+        line_number: *line_number,
+        line_type: *line_type,
+        code,
+        submatches: None,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::handlers::grep::{parse_grep_line, GrepLine, LineType};
+
+    #[test]
+    fn test_parse_grep_match() {
+        assert_eq!(
+            parse_grep_line("src/co-7-fig.rs:xxx"),
+            Some(GrepLine {
+                path: "src/co-7-fig.rs".into(),
+                line_number: None,
+                line_type: LineType::Match,
+                code: "xxx".into(),
+                submatches: None,
+            })
+        );
+        assert_eq!(
+            parse_grep_line("src/config.rs:use crate::minusplus::MinusPlus;"),
+            Some(GrepLine {
+                path: "src/config.rs".into(),
+                line_number: None,
+                line_type: LineType::Match,
+                code: "use crate::minusplus::MinusPlus;".into(),
+                submatches: None,
+            })
+        );
+        assert_eq!(
+            parse_grep_line(
+                "src/config.rs:    pub line_numbers_style_minusplus: MinusPlus<Style>,"
+            ),
+            Some(GrepLine {
+                path: "src/config.rs".into(),
+                line_number: None,
+                line_type: LineType::Match,
+                code: "    pub line_numbers_style_minusplus: MinusPlus<Style>,".into(),
+                submatches: None,
+            })
+        );
+        assert_eq!(
+            parse_grep_line("src/con-fig.rs:use crate::minusplus::MinusPlus;"),
+            Some(GrepLine {
+                path: "src/con-fig.rs".into(),
+                line_number: None,
+                line_type: LineType::Match,
+                code: "use crate::minusplus::MinusPlus;".into(),
+                submatches: None,
+            })
+        );
+        assert_eq!(
+            parse_grep_line(
+                "src/con-fig.rs:    pub line_numbers_style_minusplus: MinusPlus<Style>,"
+            ),
+            Some(GrepLine {
+                path: "src/con-fig.rs".into(),
+                line_number: None,
+                line_type: LineType::Match,
+                code: "    pub line_numbers_style_minusplus: MinusPlus<Style>,".into(),
+                submatches: None,
+            })
+        );
+        assert_eq!(
+            parse_grep_line(
+                "src/de lta.rs:pub fn delta<I>(lines: ByteLines<I>, writer: &mut dyn Write, config: &Config) -> std::io::Result<()>"
+            ),
+            Some(GrepLine {
+                path: