ux: suggest --fixed-strings flag

If a regex syntax error occurs, then ripgrep will suggest
using the --fixed-strings flag.

Fixes #727

src/args.rs

@@ -11,7 +11,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use clap;
 use encoding_rs::Encoding;
 use env_logger;
-use grep::{Grep, GrepBuilder};
+use grep::{Grep, GrepBuilder, Error as GrepError};
 use log;
 use num_cpus;
 use regex;
@@ -783,7 +783,16 @@ impl<'a> ArgMatches<'a> {
         if let Some(limit) = self.regex_size_limit()? {
             gb = gb.size_limit(limit);
         }
-        gb.build().map_err(From::from)
+        gb.build().map_err(|err| {
+            match err {
+                GrepError::Regex(err) => {
+                  let s = format!("{}\n(Hint: Try the --fixed-strings flag \
+                  to search for a literal string.)", err.to_string());
+                  From::from(s)
+                },
+                err => From::from(err)
+            }
+        })
     }
 
     /// Builds the set of glob overrides from the command line flags.