minor #6241 [ExpressionLanguage] Add caution about backslash handling (zerustech, WouterJ)

This PR was merged into the 2.7 branch.

Discussion
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[ExpressionLanguage] Add caution about backslash handling

Finishes #5576

Original description:

 > Additional backslashes are required to escape a backslash(``\``)
 > in a string or regex because a string will be stripped by the lexer.
 > This should be documented here, otherwise, user may feel confused about
 > the unexpected behavior.
 >
 > | Q             | A
 > | ------------- | ---
 > | Doc fix?      | yes
 > | New docs?     | no
 > | Applies to    | ~2.4
 > | Fixed tickets | n/a

Commits
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31d74e5 Add a caution about backslash escaping
07f8e40 [components][expression_language] Add doc for backslashes

Author: xabbuh

components/expression_language/syntax.rst

@@ -20,6 +20,18 @@ The component supports:
 * **booleans** - ``true`` and ``false``
 * **null** - ``null``
 
+.. caution::
+
+    A backslash (``\``) must be escaped by 4 backslashes (``\\\\``) in a string
+    and 8 backslashes (``\\\\\\\\``) in a regex::
+
+        echo $language->evaluate('"\\\\"'); // prints \
+        $language->evaluate('"a\\\\b" matches "/^a\\\\\\\\b$/"'); // returns true
+
+    Control characters (e.g. ``\n``) in expressions are replaced with
+    whitespace. To avoid this, escape the sequence with a single backslash
+    (e.g.  ``\\n``).
+
 .. _component-expression-objects:
 
 Working with Objects