[erlang-questions] Why is it necessary to "double-escape" [ characters in regular expressions?

[Tony Finch]

On Mon, 30 Mar 2009, David Mitchell wrote:
>
> However, "double-escaping" the [ characters (by adding a second \ prior to
> the [ character) does exactly what I want:
>   re:replace("abc123 <![CDATA[< abc123", "<!\\[CDATA\\[<", "<", [{return, list}])
> returns "abc123 < abc123", which is the result I'm after.

There are two levels of backslash escaping here, one for string literals
and one for regular expressions. The \\ in the string literal becomes \ in
the run-time string, which the regex implementation treats as an escape
character for the following [. If you only write \[ in the string literal
then this becomes [ in the run-time string which the regex implementation
treats as the start of a character class specifier, and since there's no
closing ] it throws a syntax error.

Tony.
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