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[components][expression_language] Add doc for backslashes #5576

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[components][expression_language] Add doc for backslashes #5576

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22 changes: 22 additions & 0 deletions components/expression_language/syntax.rst
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Expand Up @@ -181,6 +181,28 @@ Comparison Operators
You must use parenthesis because the unary operator ``not`` has precedence
over the binary operator ``matches``.

A backslash(``\``) must be escaped by 4 backslashes(``\\\\``) in a string and
8 backslashes(``\\\\\\\\``) in a regex::

$language->evaluate('"\\\\"');
// returns \

$language->evaluate('"a\\\\b" matches "/^a\\\\\\\\b$/"');
// returns true

Control characters must be defined as the escaped form of their escape sequences.
Otherwise, they will be replaced by spaces and ignored::

$language->evaluate('"a\nb"');
// returns a b

$language->evaluate('"a\\nb"');
// returns a\nb

This is because the backslashes in a string will be stripped by the
``stripcslashes()`` function and the stripped slashes in a regex will be
stripped again by the regex engine.

Examples::

$ret1 = $language->evaluate(
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