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< Erlang ~ Quirks in Erlang R14B binaries |
| LRP |
 Posted: Mon May 09, 2011 3:53 am |
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Joined: 08 Oct 2008
Posts: 8
Location: Boston, MA.
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Or maybe I just don't know what I'm doing.
I've spent days trying to use regular expressions to filter binaries with scant success. Here are two experiments that illustrate some of the frustrations I've encountered.
Can some kind soul either show me how to correct my mistakes or, if it so be, confirm that there are quirks in Erlang R14B binaries.
Latex produces typeset text output. Among other things, it outputs both opening and closing quotation marks, consistent with professional typesetting convention. Open quotes in Latex input files, (*.tex), are represented as shown below:
To `quote' in Latex
To ``quote'' in Latex
To ``quote" in Latex
Our goal is to filter and convert a binary derived from a *.txt file to produce proper *.tex markup.
Wierdness 1
Here's a quote represented as a binary:
A = <<"\"The quick brown fox\"">>.
<<"\"The quick brown fox.\"">>
Which displays as we'd expect:
io:format(A,[]).
"The quick brown fox."ok
We can use a regular expression to convert the opening quote to *.tex markup:
B = binary:replace(A,<<"\"">>,<<"``">>).
<<"``The quick brown fox.\"">>
Which also displays as we'd expect:
io:format(B,[]).
``The quick brown fox."ok
But we need to convert opening quotes only which, presumably, can be identified as a double quote character (") followed by a Perl "word" character (\w).
D = binary:replace(A,<<"\"\w">>,<<"``\w">>, [global]).
<<"\"The quick brown fox.\"">>
Display confirms our fears:
io:format(G,[]).
"The quick brown fox."ok
Maybe re will stand us in better stead.
E = re:replace(A,<<"\"">>,<<"``">>, [global,{return, binary}]).
<<"``The quick brown fox.``">>
So far so good. Let's try for opening quotes only:
F = re:replace(A,<<"\"\w">>,<<"``\w">>, [global,{return, binary}]).
<<"\"The quick brown fox.\"">>
io:format(A,[]).
"The quick brown fox."ok
Grrrr.
Wierdness 2
We would like to delete all newlines at the beginning of our binary.
If we know how many, it's easy:
A = <<"\n\n\n\nThe quick brown fox">>.
<<"\n\n\n\nThe quick brown fox">>
B = binary:replace(A,<<"\n\n\n\n">>,<<>>).
<<"The quick brown fox">>
But, following Perl-style regular expressions documented here:
http://www.troubleshooters.com/codecorn/littperl/perlreg.htm
We should be able to delete an arbitrary number of newlines:
C = binary:replace(A,<<"\n{0,}">>,<<>>).
<<"\n\n\n\nThe quick brown fox">>
NOT.
Maybe re can help us out:
5> E = re:replace(A,<<"\n{0, }">>,<<>>, [{return, binary}]).
<<"\n\n\n\nThe quick brown fox">>
NOT. Grrrr.
So what's going on here? Is there a better way? |
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| rvirding |
| Posted: Mon May 09, 2011 3:56 pm |
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Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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First to make some points:
- In Erlang strings and binary strings '\' is the quote character so to get it in the actual string you need to quote it as well. So <<"\"\w">> is actually <<"\"w>>, to get the '\' in the string you need to write <<"\"\\w">>.
- In the module binary patterns are NOT regular expressions but literal binaries. So the pattern <<"\"\\w">> will match against the actual character sequence "\w. To use regular expressions you need to use the module re.
- In re:replace the replacement string/binary is inserted as is except for the character '&' which is replaced by the whole matching string (quote it to get it in literally) or \1-\9 which are replaced by the matching sub-expressions in the pattern.
- You can use instead '*' instead of '{0,}', it means the same thing and is the standard way of expressing "zero or more". '+' is the standard way for "one or more".
- If you want to make sure that you only work on the beginning of the string then you need to anchor your regular expression with '^'. Check in the 're' docs how the option 'multiline' affects the effect of '^' and '$'. I think the default value is probably what you need.
So putting all this together this is what I found running R14B02:
Code: A = <<"\"The quick\">>.
40> binary:replace(A, <<"\"">>, <<"``">>).
<<"``The quick\"">>
43> re:replace(A, <<"\"\w">>, <<"``\w">>, [{return,binary}]).
<<"\"The quick\"">>
44> re:replace(A, <<"\"\\w">>, <<"``\w">>, [{return,binary}]).
<<"``whe quick\"">>
45> re:replace(A, <<"\"(\\w)">>, <<"``\\1">>, [{return,binary}]).
<<"``The quick\"">>
I did not have the same problems you did when trying to remove newlines at the beginning.
Code: B = <<"\n\n\n\nThe quick">>.
C = <<"\n\n\n\nThe quick\n\n">>.
55> binary:replace(B, <<"\n">>, <<>>, [global]).
<<"The quick">>
56> binary:replace(C, <<"\n">>, <<>>, [global]).
<<"The quick">>
57> re:replace(B, <<"\n*">>, <<>>, [{return,binary},global]).
<<"The quick">>
58> re:replace(C, <<"\n*">>, <<>>, [{return,binary},global]).
<<"The quick">>
59> re:replace(B, <<"^\n*">>, <<>>, [{return,binary},global]).
<<"The quick">>
60> re:replace(C, <<"^\n*">>, <<>>, [{return,binary},global]).
<<"The quick\n\n">>
I hope this helps a little. |
_________________
Robert Virding, Erlang Solutions Ltd. |
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| wuji |
| Posted: Mon Sep 10, 2012 8:57 am |
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Joined: 10 Aug 2012
Posts: 654
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| dongdongwu |
| Posted: Wed Sep 19, 2012 8:15 am |
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