Regex to to parse csv with nested quotes [duplicate]

Answer 1

I use regexp from this blog article which is about the same problem you are trying to solve.

See it here: http://www.kimgentes.com/worshiptech-web-tools-page/2008/10/14/regex-pattern-for-parsing-csv-files-with-embedded-commas-dou.html

In short ^(("(?:[^"]|"")*"|[^,]*)(,("(?:[^"]|"")*"|[^,]*))*)$

Answer 2

OK, you've seen from the comments that regex is so not the right tool for this. But if you insist, here goes:

This regex will work in Java (or .NET and other implementations that support possessive quantifiers and verbose regexes):

^            # Start of string
(?:          # Match the following:
 (?:         #  Either match
  [^",\n]*+  #   0 or more characters except comma, quote or newline
 |           #  or
  "          #   an opening quote
  (?:        #   followed by either
   [^"]*+    #    0 or more non-quote characters
  |          #   or
   ""        #    an escaped quote ("")
  )*         #   any number of times
  "          #   followed by a closing quote
 )           #  End of alternation
 ,           #  Match a comma (separating the CSV columns)
)*           # Do this zero or more times.
(?:          # Then match
 (?:         #  using the same rules as above
  [^",\n]*+  #  an unquoted CSV field
 |           #  or a quoted CSV field
  "(?:[^"]*+|"")*"
 )           #  End of alternation
)            # End of non-capturing group
$            # End of string

Java code:

boolean foundMatch = subjectString.matches(
    "(?x)^         # Start of string\n" +
    "(?:           # Match the following:\n" +
    " (?:          #  Either match\n" +
    "  [^\",\\n]*+ #   0 or more characters except comma, quote or newline\n" +
    " |            #  or\n" +
    "  \"          #   an opening quote\n" +
    "  (?:         #   followed by either\n" +
    "   [^\"]*+    #    0 or more non-quote characters\n" +
    "  |           #   or\n" +
    "   \"\"       #    an escaped quote (\"\")\n" +
    "  )*          #   any number of times\n" +
    "  \"          #   followed by a closing quote\n" +
    " )            #  End of alternation\n" +
    " ,            #  Match a comma (separating the CSV columns)\n" +
    ")*            # Do this zero or more times.\n" +
    "(?:           # Then match\n" +
    " (?:          #  using the same rules as above\n" +
    "  [^\",\\n]*+ #  an unquoted CSV field\n" +
    " |            #  or a quoted CSV field\n" +
    "  \"(?:[^\"]*+|\"\")*\"\n" +
    " )            #  End of alternation\n" +
    ")             # End of non-capturing group\n" +
    "$             # End of string");

Be aware that you can't assume that every line in a CSV file is a complete row. You can have newlines within a CSV row (as long as the column containing the newlines is enclosed in quotes). This regex knows this, but it will fail if you feed it only a partial row. Which is yet another reason why you really need a CSV parser to validate a CSV file. That's what parsers do. If you control your input and know that you'll never have newlines inside a CSV field, you might get away with it, but only then.

Answer 3

I haven't done Java in a while, so here's a pseudocode to do this. You could use this as a function that accepts a String representing a row of your csv.

1. Split the row by "'" delimiter into an array of strings. (method might be called string.split())
2. Iterate through the array (cells).
    3. If the current string (cell) contains a double quote:
        4. If it doesn't start with a quote - return false; else remove that quote
        5. If it doesn't end with a quote - return false; else remove that quote
        6. Iterate through the remaining characters of the string
            7. If a quote is found, check if the next character is also a quote - if it is not - return false
        7. End character iteration
    8. End if
9. End array iteration
10. Return true