Add clarification of the r string modifier

doc/data/messages/a/anomalous-backslash-in-string/details.rst

@@ -1,2 +1,25 @@
 ``\z`` is same as ``\\z`` because there's no escape sequence for ``z``. But it is not clear
 for the reader of the code.
+
+Background
+
+Python's string literals use the backslash for their own escape
+sequences (like \n for a newline). When Python sees an escape sequence
+it doesn't recognize, such as "\." (a literal "dot" character in regex),
+it gives a DeprecationWarning.
+
+To avoid this warning, you can use a raw string literal for your regular
+expression. Raw string literals don't treat the backslash as a special
+character and are often used for regular expressions in Python.
+
+By adding the r before the string, you're telling Python to treat this
+as a raw string literal, so it won't try to interpret \. as an escape
+sequence and will instead ignore it.
+
+An alternative would be to use double backslash \\. The first backslash
+escapes the second one when the string is parsed by python. The second
+backslash works as the escape sequence for the . when sent to the regex
+engine for parsing. In other words, '\\.' becomes '\.' when parsed by
+python, and it can then be used in a regex. The "r" before the string
+tells python to treat the string as a "raw" string literal (and not to
+modify it).