From c7858f61fcd1062ab2333f286ccec142b9c916e3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Israel=20Saeta=20P=C3=A9rez?= <dukebody@gmail.com>
Date: Sun, 3 Apr 2016 15:23:22 +0200
Subject: [PATCH] DOC: Clarify when csv separator is being parsed as regex.
 Resolves #10208.

---
 doc/source/io.rst    | 7 ++++---
 pandas/io/parsers.py | 9 ++++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/doc/source/io.rst b/doc/source/io.rst
index d606e919e4292..a78222dd748ad 100644
--- a/doc/source/io.rst
+++ b/doc/source/io.rst
@@ -91,9 +91,10 @@ filepath_or_buffer : various
   :class:`~python:io.StringIO`).
 sep : str, defaults to ``','`` for :func:`read_csv`, ``\t`` for :func:`read_table`
   Delimiter to use. If sep is ``None``,
-  will try to automatically determine this. Regular expressions are accepted,
-  use of a regular expression will force use of the python parsing engine and
-  will ignore quotes in the data.
+  will try to automatically determine this. Separators longer than 1 character
+  and different from ``'\s+'`` will be interpreted as regular expressions, will
+  force use of the python parsing engine and will ignore quotes in the data.
+  Regex example: ``'\\r\\t'``.
 delimiter : str, default ``None``
   Alternative argument name for sep.
 
diff --git a/pandas/io/parsers.py b/pandas/io/parsers.py
index e99fc3db606af..7bd8a593661c5 100755
--- a/pandas/io/parsers.py
+++ b/pandas/io/parsers.py
@@ -221,8 +221,9 @@ class ParserWarning(Warning):
 
 _sep_doc = """sep : str, default {default}
     Delimiter to use. If sep is None, will try to automatically determine
-    this. Regular expressions are accepted and will force use of the python
-    parsing engine and will ignore quotes in the data."""
+    this. Separators longer than 1 character and different from '\s+' will be
+    interpreted as regular expressions, will force use of the python parsing
+    engine and will ignore quotes in the data. Regex example: '\\r\\t'"""
 
 _read_csv_doc = """
 Read CSV (comma-separated) file into DataFrame
@@ -674,7 +675,9 @@ def _clean_options(self, options, engine):
             elif engine not in ('python', 'python-fwf'):
                 # wait until regex engine integrated
                 fallback_reason = "the 'c' engine does not support"\
-                                  " regex separators"
+                                  " regex separators (separators > 1 char and"\
+                                  " different from '\s+' are"\
+                                  " interpreted as regex)"
                 engine = 'python'
 
         if fallback_reason and engine_specified: