diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 3b042db624..994eb9159a 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -53,6 +53,18 @@ For shell scripts specifically (not exhaustive):
  - We do not write the noiseword "function" in front of shell
    functions.
 
+ - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
+   [::], [==], nor [..]) for portability.
+
+   - We do not use \{m,n\};
+
+   - We do not use -E;
+
+   - We do not use ? nor + (which are \{0,1\} and \{1,\}
+     respectively in BRE) but that goes without saying as these
+     are ERE elements not BRE (note that \? and \+ are not even part
+     of BRE -- making them accessible from BRE is a GNU extension).
+
 For C programs:
 
  - We use tabs to indent, and interpret tabs as taking up to