Question

I am trying to write a class that can parse an iCalendar file and am hitting some brick walls. Each line can be in the format:

PARAMETER[;PARAM_PROPERTY..]:VALUE[,VALUE2..]

It’s pretty easy to parse with either a bunch of splits or regex’s until you find out that values can have backticked commas, also they can be double quote marked which makes life hard. for example:

PARAMETER:"my , cool, value",value\,2,value3

In this example you are meant to pull out the three values:

• my , cool value
• value,2
• value3

Which makes it a little more difficult.

Suggestions?

Answer 1

Go through the file char by char and split the values manually, whenever you have a quotation mark you enter “quotation mode” where you won’t split at commas and when the closing quotation mark comes you leave it.

For the backticked commas: If you read in a backslash you also read the next character and decide what to do with it then.

Of course that’s not extremely efficient, but you can’t use regular expressions for this. I mean you can, but since I believe that there also can be escaped quotation marks this is going to be really messy.

If you want to give it a try though:

• let’s start by matching a quotation mark followed by characters that are not: "[^"]*"
• to overcome the problem of escaped characters you can use lookaheads (?<!\\)"[^"]*(?<!\\)"
• now it will break if escaped quotation marks are in the value, maybe this works?(haven’t tested it) (?<!\\)"[^"|(?<=\\)"]*(?<!\\)"

So you see it very fast get’s messy, so I would suggest to you to read it in characterwise.