regex of "${endpoint}[0-9a-z]" changed for robot 3.1

In 'Get Endpoint Paths' a regular expression containing the following substring
was mis-interpreted by robot 3.1:

${endpoint}[0-9a-z]

This resulted in an error like this:

Variable '${endpoint}' is string, not list or dictionary, and thus accessing
item '0-9a-z' from it is not possible.

The fix is to add a backslash to escape the left square bracket.

${endpoint}\[0-9a-z]

Change-Id: I06a2f5fa3903579e80e498cc1085799a4cc15eb5
Signed-off-by: Michael Walsh <micwalsh@us.ibm.com>
diff --git a/lib/utils.robot b/lib/utils.robot
index 81b82a2..5847ced 100755
--- a/lib/utils.robot
+++ b/lib/utils.robot
@@ -957,7 +957,7 @@
     # For a given string, look for prefix and suffix for matching expression.
     # Start of string followed by zero or more of any character followed by
     # any digit or lower case character.
-    ${resp}=  Get Matches  ${list}  regexp=^.*[0-9a-z_].${endpoint}[0-9a-z]*$
+    ${resp}=  Get Matches  ${list}  regexp=^.*[0-9a-z_].${endpoint}\[0-9a-z]*$
 
     [Return]  ${resp}