Add example to -r/--replace docs.

Fixes #308

doc/rg.1

@@ -340,6 +340,17 @@ Neither this flag nor any other flag will modify your files.
 .PP
 Capture group indices (e.g., $5) and names (e.g., $foo) are supported in
 the replacement string.
+.PP
+Note that the replacement by default replaces each match, and NOT the
+entire line.
+To replace the entire line, you should match the entire line.
+For example, to emit only the first phone numbers in each line:
+.IP
+.nf
+\f[C]
+rg\ \[aq]^.*([0\-9]{3}\-[0\-9]{3}\-[0\-9]{4}).*$\[aq]\ \-\-replace\ \[aq]$1\[aq]
+\f[]
+.fi
 .RE
 .TP
 .B \-s, \-\-case\-sensitive

doc/rg.1.md

@@ -231,6 +231,12 @@ Project home page: https://github.com/BurntSushi/ripgrep
     Capture group indices (e.g., $5) and names (e.g., $foo) are supported
     in the replacement string.
 
+    Note that the replacement by default replaces each match, and NOT the
+    entire line. To replace the entire line, you should match the entire line.
+    For example, to emit only the first phone numbers in each line:
+
+        rg '^.*([0-9]{3}-[0-9]{3}-[0-9]{4}).*$' --replace '$1'
+
 -s, --case-sensitive
 : Search case sensitively. This overrides --ignore-case and --smart-case.
 

src/app.rs

@@ -417,7 +417,10 @@ lazy_static! {
              "Replace every match with the string given when printing \
               results. Neither this flag nor any other flag will modify your \
               files.\n\nCapture group indices (e.g., $5) and names \
-              (e.g., $foo) are supported in the replacement string.");
+              (e.g., $foo) are supported in the replacement string.\n\n\
+              Note that the replacement by default replaces each match, and \
+              NOT the entire line. To replace the entire line, you should \
+              match the entire line.");
         doc!(h, "case-sensitive",
              "Search case sensitively.",
              "Search case sensitively. This overrides -i/--ignore-case and \