Regular Expression Fill-Down

Question

I have a plain text file something like this:

Ford\tTaurus
  F-150
  F-250
Toyota\tCamry
  Corsica

In other words, a two-level hierarchy where the first child is on the same line as the parent, but subsequent children on lines following, distinguished from being a parent by a two-space prefix (\t above represents a literal tab in the text).

I need to convert to this using RegEx:

Ford\tTaurus
Ford\tF-150
Ford\tF-250
Toyota\tCamry
Toyota\tCorsica

So, I need to capture the parent (text between \r\n and \t not starting with \s\s), and apply that in the middle of any \r\n\s\s found until the next parent.

I have a feeling this can be done with some sort of nested groups, but I think I need more caffeine or something, can't seem to work out the pattern.

(Using .NET with IgnoreWhitespace off and Multiline off)

Answer 1

Any particular reason you want to use regular expressions for this? Here's code which does what I think you want, without bothering to work out regular expressions:

using System;
using System.IO;

class Test
{
    static void Main(string[] args)
    {
        string currentManufacturer = null;

        using (TextReader reader = File.OpenText(args[0]))
        using (TextWriter writer = File.CreateText(args[1]))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
            {
                string car;
                if (line.StartsWith("  "))
                {
                    if (currentManufacturer == null)
                    {
                        // Handle this properly in reality :)
                        throw new Exception("Invalid data");
                    }
                    car = line.Substring(2);
                }
                else
                {
                    string[] bits = line.Split('\t');
                    if (bits.Length != 2)
                    {
                        // Handle this properly in reality :)
                        throw new Exception("Invalid data");
                    }
                    currentManufacturer = bits[0];
                    car = bits[1];
                }
                writer.WriteLine("{0}\t{1}", currentManufacturer, car);
            }
        }
    }
}

Answer 2

It is simple (but not wise or fast) to achieve this by using regular expressions.

Replace

(?<=^(Ford\t|Toyota\t).*?)^  

with $1. Make sure ^ and $ match at line beginnings/endings and . matches newline.