Make backreference support clear.

Fixes #268.

README.md

@@ -1,9 +1,15 @@
 ripgrep (rg)
 ------------
-`ripgrep` is a command line search tool that combines the usability of The
-Silver Searcher (an `ack` clone) with the raw speed of GNU grep. `ripgrep` has
-first class support on Windows, Mac and Linux, with binary downloads available
-for [every release](https://github.com/BurntSushi/ripgrep/releases).
+`ripgrep` is a line oriented search tool that combines the usability of The
+Silver Searcher (an `ack` clone) with the raw speed of GNU grep. `ripgrep`
+works by recursively searching your current directory for a regex pattern.
+`ripgrep` has first class support on Windows, Mac and Linux, with binary
+downloads available for
+[every release](https://github.com/BurntSushi/ripgrep/releases).
+
+ripgrep's regex engine uses finite automata and guarantees linear time
+searching. Because of this, features like backreferences and arbitrary
+lookaround are not supported.
 
 [![Linux build status](https://travis-ci.org/BurntSushi/ripgrep.svg?branch=master)](https://travis-ci.org/BurntSushi/ripgrep)
 [![Windows build status](https://ci.appveyor.com/api/projects/status/github/BurntSushi/ripgrep?svg=true)](https://ci.appveyor.com/project/BurntSushi/ripgrep)

doc/rg.1

@@ -24,6 +24,11 @@ rg [\f[I]options\f[]] \-\-version
 ripgrep (rg) combines the usability of The Silver Searcher (an ack
 clone) with the raw speed of grep.
 .PP
+ripgrep\[aq]s regex engine uses finite automata and guarantees linear
+time searching.
+Because of this, features like backreferences and arbitrary lookaround
+are not supported.
+.PP
 Project home page: https://github.com/BurntSushi/ripgrep
 .SH COMMON OPTIONS
 .TP

doc/rg.1.md

@@ -21,6 +21,10 @@ rg [*options*] --version
 ripgrep (rg) combines the usability of The Silver Searcher (an ack clone) with
 the raw speed of grep.
 
+ripgrep's regex engine uses finite automata and guarantees linear time
+searching. Because of this, features like backreferences and arbitrary
+lookaround are not supported.
+
 Project home page: https://github.com/BurntSushi/ripgrep
 
 # COMMON OPTIONS

src/app.rs

@@ -5,6 +5,10 @@ use clap::{App, AppSettings, Arg, ArgSettings};
 const ABOUT: &'static str = "
 ripgrep (rg) recursively searches your current directory for a regex pattern.
 
+ripgrep's regex engine uses finite automata and guarantees linear time
+searching. Because of this, features like backreferences and arbitrary
+lookaround are not supported.
+
 Project home page: https://github.com/BurntSushi/ripgrep
 
 Use -h for short descriptions and --help for more details.";