How do I robustly parse malformed CSV?

Question

I'm processing data from government sources (FEC, state voter databases, etc). It's inconsistently malformed, which breaks my CSV parser in all sorts of delightful ways.

It's externally sourced and authoritative. I must parse it, and I cannot have it re-input, validated on input, or the like. It is what it is; I don't control the input.

Properties:

1. Fields contain malformed UTF-8 (e.g. Foo \xAB bar)
2. The first field of a line specifies the record type from a known set. Knowing the record type, you know how many fields there are and their respective data types, but not until you do.
3. Any given line within a file might use quoted strings ("foo",123,"bar") or unquoted (foo,123,bar). I haven't yet encountered any where it's mixed within a given line (i.e. "foo",123,bar) but it's probably in there.
4. Strings may include internal newline, quote, and/or comma character(s).
5. Strings may include comma separated numbers.
6. Data files can be very large (millions of rows), so this needs to still be reasonably fast.

I'm using Ruby FasterCSV (known as just CSV in 1.9), but the question should be language-agnostic.

My guess is that a solution will require preprocessing substitution with unambiguous record separator / quote characters (eg ASCII RS, STX). I've started a bit here but it doesn't work for everything I get.

How can I process this kind of dirty data robustly?

ETA: Here's a simplified example of what may be in single file:

"this","is",123,"a","normal","line"
"line","with "an" internal","quote"
"short line","with
an
"internal quote", 1 comma and
linebreaks"
un "quot" ed,text,with,1,2,3,numbers
"quoted","number","series","1,2,3"
"invalid \xAB utf-8"

Answer 1

It is possible to subclass Ruby's File to process each line of the the CSV file before it is passed to the Ruby's CSV parser. For example, here's how I used this trick to replace non-standard backslash-escaped quotes \" with standard double-quotes ""

class MyFile < File
  def gets(*args)
    line = super
    if line != nil
      line.gsub!('\\"','""')  # fix the \" that would otherwise cause a parse error
    end
    line
  end
end

infile = MyFile.open(filename)
incsv = CSV.new(infile)

while row = incsv.shift
  # process each row here
end

You could in principle do all sorts of additional processing, e.g. UTF-8 cleanups. The nice thing about this approach is you handle the file on a line by line basis, so you don't need to load it all into memory or create an intermediate file.

Answer 2

First, here is a rather naive attempt: http://rubular.com/r/gvh3BJaNTc

/"(.*?)"(?=[\r\n,]|$)|([^,"\s].*?)(?=[\r\n,]|$)/m

The assumptions here are:

• A field may start with quotes. In which case, it should end with a quote that is either:
  • before a comma
  • before a new line (if it is last field on its line)
  • before the end of the file (if it is last field on the last line)
• Or, its first character is not a quote, so it contains characters until the same condition as before is met.

This almost does what you want, but fails on these fields:

1 comma and
linebreaks"

As TC had pointed out in the comments, your text is ambiguous. I'm sure you already know it, but for completeness:

• "a" - is that a or "a"? How do you represent a value that you want to be wrapped in quotes?
• "1","2" - might be parsed as 1,2, or as 1","2 - both are legal.
• ,1 \n 2, - End of line, or newline in the value? You cannot tell, specially if this is supposed to be the last value of its line.
• 1 \n 2 \n 3 - One value with newlines? Two values (1\n2,3 or 1,2\n3)? Three values?

You may be able to get some clues if you examine the first value on each row, which as you have said, should tell you the number of columns and their types - this can give you the additional information you are missing to parse the file (for example, if you know there should another field in this line, then all newlines belong in the current value). Even then though, it looks like there are serious problems here...