[erlang-bugs] Module re, escape needs double backslash

[Erik Søe Sørensen]

Not a bug, just a surprise -
The string
"/\\*.*?\\*/"
does only contain two backslashes:

     > length([C || C <- "/\\*.*?\\*/", C==92]). % The character code
    for backslash being 92.
    2

The string literal in the *Erlang source file*[1], of course, plainly 
contains four of them.
The surprise is that there are two interpreters involved -- two layers, 
each of which requires backslash-escaping:

 1. The Erlang parser reads [Backslash, Backslash] and puts a single
    backslash into the string literal (rather than taking the second
    backslash as a signal to start an escape sequence).
 2. The "re" module reads [Backslash, Asterisk] and interprets this by
    taking the asterisk literally (rather than as a zero-or-more modifier).


Or going the other way: If we desire a literal asterisk in the pattern, 
we must escape it, so that "re" sees a backslash in front of the asterisk.
Thus, we want a backslash in the string. And if we want to write that as 
a string literal, then in order for the string to contain a backslash, 
we must put another backslash in front of it in the Erlang source code, 
because backslash means something special to the Erlang parser as well.

/Erik

[1] Or in this case, the expression typed into the Erlang shell.

On 20-11-2012 09:17, Arif Ishaq wrote:
> Hi,
> The backslash character as escape doesn't work as expected.
> Erlang R15B (erts-5.9) [smp:4:4] [async-threads:0]
> Eshell V5.9  (abort with ^G)
> 1> String = "/* this is a C comment */".
> "/* this is a C comment */"
> 2> re:run(String, "/\*.*?\*/").
> ** exception error: bad argument
>      in function  re:run/2
>         called as re:run("/* this is a C comment */","/*.*?*/")
> 3> re:run(String, "/\\*.*?\\*/").
> {match,[{0,25}]}
> 4>
> Best regards
> PS. The documentation in "erl5.9/lib/stdlib-1.18/doc/html/re.html" says:
> ".. the pattern
> /\*.*?\*/
> does the right thing with the C comments."


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