Implement -o, --only-matching.

Fixes BurntSushi#34.

src/args.rs

@@ -165,6 +165,10 @@ Less common options:
         a list of matching files such as with --count, --files-with-matches
         and --files.
 
+    -o, --only-matching
+        Only print the text that matches the regex, with each match on a
+        separate line.
+
     -p, --pretty
         Alias for --color=always --heading -n.
 
@@ -236,6 +240,7 @@ pub struct RawArgs {
     flag_no_mmap: bool,
     flag_no_filename: bool,
     flag_null: bool,
+    flag_only_matching: bool,
     flag_pretty: bool,
     flag_quiet: bool,
     flag_regexp: Vec<String>,
@@ -283,6 +288,7 @@ pub struct Args {
     no_ignore_parent: bool,
     no_ignore_vcs: bool,
     null: bool,
+    only_matching: bool,
     quiet: bool,
     replace: Option<Vec<u8>>,
     text: bool,
@@ -415,6 +421,7 @@ impl RawArgs {
                 // --no-ignore implies --no-ignore-vcs
                 self.flag_no_ignore_vcs || no_ignore,
             null: self.flag_null,
+            only_matching: self.flag_only_matching,
             quiet: self.flag_quiet,
             replace: self.flag_replace.clone().map(|s| s.into_bytes()),
             text: text,
@@ -570,7 +577,8 @@ impl Args {
             .line_per_match(self.line_per_match)
             .quiet(self.quiet)
             .null(self.null)
-            .with_filename(self.with_filename);
+            .with_filename(self.with_filename)
+            .only_matching(self.only_matching);
         if let Some(ref rep) = self.replace {
             p = p.replace(rep.clone());
         }

src/printer.rs

@@ -42,6 +42,8 @@ pub struct Printer<W> {
     replace: Option<Vec<u8>>,
     /// Whether to prefix each match with the corresponding file name.
     with_filename: bool,
+    /// Whether to only print matching text.
+    only_matching: bool,
 
     /// The choice of Colours
     color_choice: ColorChoice
@@ -90,6 +92,7 @@ impl<W: Terminal + Send> Printer<W> {
             null: false,
             replace: None,
             with_filename: false,
+            only_matching: false,
             color_choice: ColorChoice::new()
         }
     }
@@ -163,6 +166,13 @@ impl<W: Terminal + Send> Printer<W> {
         self
     }
 
+    /// When set, only the text that matches the regex is printed, with each match on a
+    /// separate line.
+    pub fn only_matching(mut self, yes: bool) -> Printer<W> {
+        self.only_matching = yes;
+        self
+    }
+
     /// Returns true if and only if something has been printed.
     pub fn has_printed(&self) -> bool {
         self.has_printed
@@ -242,6 +252,13 @@ impl<W: Terminal + Send> Printer<W> {
         end: usize,
         line_number: Option<u64>,
     ) {
+        if self.only_matching {
+            for (match_start, match_end) in re.find_iter(&buf[start..end]) {
+                self.write(&buf[start + match_start..start + match_end]);
+                self.write_eol();
+            }
+            return;
+        }
         if !self.line_per_match {
             let column =
                 if self.column {

tests/tests.rs

@@ -925,3 +925,17 @@ fn type_list() {
     // This can change over time, so just make sure we print something.
     assert!(!lines.is_empty());
 }
+
+// See: https://github.com/BurntSushi/ripgrep/issues/34
+sherlock!(only_matching, r"of \w+", |wd: WorkDir, mut cmd: Command| {
+    cmd.arg("--only-matching");
+    let lines: String = wd.stdout(&mut cmd);
+    let expected = "\
+of this
+of detective
+of luck
+of straw
+of cigar
+";
+    assert_eq!(lines, expected);
+});