Add `-o/--only-matching` flag.

Currently, the `--only-matching` flag conflicts with the `--replace`
flag. In the future, this restriction may be relaxed.

Fixes #34

3 files changed, 25 insertions, 2 deletions

diff --git a/src/app.rs b/src/app.rs
index f19ff8cf..94804828 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -144,6 +144,7 @@ pub fn app() -> App<'static, 'static> {
         .arg(flag("no-ignore-parent"))
         .arg(flag("no-ignore-vcs"))
         .arg(flag("null").short("0"))
+        .arg(flag("only-matching").short("o").conflicts_with("replace"))
         .arg(flag("path-separator").value_name("SEPARATOR").takes_value(true))
         .arg(flag("pretty").short("p"))
         .arg(flag("replace").short("r").value_name("ARG").takes_value(true))
@@ -421,6 +422,10 @@ lazy_static! {
               printing a list of matching files such as with --count, \
               --files-with-matches and --files. This option is useful for use \
               with xargs.");
+        doc!(h, "only-matching",
+             "Print only matched parts of a line.",
+             "Print only the matched (non-empty) parts of a matching line, \
+              with each such part on a separate output line.");
         doc!(h, "path-separator",
              "Path separator to use when printing file paths.",
              "The path separator to use when printing file paths. This \
diff --git a/src/args.rs b/src/args.rs
index 90e5a9a4..34f38792 100644
--- a/src/args.rs
+++ b/src/args.rs
@@ -65,6 +65,7 @@ pub struct Args {
     no_ignore_vcs: bool,
     no_messages: bool,
     null: bool,
+    only_matching: bool,
     path_separator: Option<u8>,
     quiet: bool,
     quiet_matched: QuietMatched,
@@ -141,6 +142,7 @@ impl Args {
             .heading(self.heading)
             .line_per_match(self.line_per_match)
             .null(self.null)
+            .only_matching(self.only_matching)
             .path_separator(self.path_separator)
             .with_filename(self.with_filename)
             .max_columns(self.max_columns);
@@ -345,6 +347,7 @@ impl<'a> ArgMatches<'a> {
             no_ignore_vcs: self.no_ignore_vcs(),
             no_messages: self.is_present("no-messages"),
             null: self.is_present("null"),
+            only_matching: self.is_present("only-matching"),
             path_separator: try!(self.path_separator()),
             quiet: quiet,
             quiet_matched: QuietMatched::new(quiet),
diff --git a/src/printer.rs b/src/printer.rs
index c1fc0eb8..c624da2a 100644
--- a/src/printer.rs
+++ b/src/printer.rs
@@ -58,6 +58,8 @@ pub struct Printer<W> {
     /// Whether to print NUL bytes after a file path instead of new lines
     /// or `:`.
     null: bool,
+    /// Print only the matched (non-empty) parts of a matching line
+    only_matching: bool,
     /// A string to use as a replacement of each match in a matching line.
     replace: Option<Vec<u8>>,
     /// Whether to prefix each match with the corresponding file name.
@@ -83,6 +85,7 @@ impl<W: WriteColor> Printer<W> {
             heading: false,
             line_per_match: false,
             null: false,
+            only_matching: false,
             replace: None,
             with_filename: false,
             colors: ColorSpecs::default(),
@@ -144,6 +147,12 @@ impl<W: WriteColor> Printer<W> {
         self
     }
 
+    /// Print only the matched (non-empty) parts of a matching line
+    pub fn only_matching(mut self, yes: bool) -> Printer<W> {
+        self.only_matching = yes;
+        self
+    }
+
     /// A separator to use when printing file paths. When empty, use the
     /// default separator for the current platform. (/ on Unix, \ on Windows.)
     pub fn path_separator(mut self, sep: Option<u8>) -> Printer<W> {
@@ -232,7 +241,7 @@ impl<W: WriteColor> Printer<W> {
         end: usize,
         line_number: Option<u64>,
     ) {
-        if !self.line_per_match {
+        if !self.line_per_match && !self.only_matching {
             let column =
                 if self.column {
                     Some(re.find(&buf[start..end])
@@ -298,7 +307,13 @@ impl<W: WriteColor> Printer<W> {
                 self.write_eol();
             }
         } else {
-            self.write_matched_line(re, &buf[start..end]);
+            let line_buf = if self.only_matching {
+                let m = re.find(&buf[start..end]).unwrap();
+                &buf[start + m.start()..start + m.end()]
+            } else {
+                &buf[start..end]
+            };
+            self.write_matched_line(re, line_buf);
             // write_matched_line guarantees to write a newline.
         }
     }