regex - Regular Expression over multiple lines

Question

I'm stuck with this for several hours now and cycled through a wealth of different tools to get the job done. Without success. It would be fantastic, if someone could help me out with this.

Here is the problem:

I have a very large CSV file (400mb+) that is not formatted correctly. Right now it looks something like this:

This is a long abstract describing something. What follows is the tile for this sentence."   
,Title1  
This is another sentence that is running on one line. On the next line you can find the title.   
,Title2

As you can probably see the titles ",Title1" and ",Title2" should actually be on the same line as the foregoing sentence. Then it would look something like this:

This is a long abstract describing something. What follows is the tile for this sentence.",Title1  
This is another sentence that is running on one line. On the next line you can find the title.,Title2

Please note that the end of the sentence can contain quotes or not. In the end they should be replaced too.

Here is what I came up with so far:

sed -n '1h;1!H;${;g;s/."?.*,//g;p;}' out.csv > out1.csv

This should actually get the job done of matching the expression over multiple lines. Unfortunately it doesn't :)

The expression is looking for the dot at the end of the sentence and the optional quotes plus a newline character that I'm trying to match with .*.

Help much appreciated. And it doesn't really matter what tool gets the job done (awk, perl, sed, tr, etc.).

Answer 1

Multiline in sed isn't necessarily tricky per se, it's just that it uses commands most people aren't familiar with and have certain side effects, like delimiting the current line from the next line with a ' ' when you use 'N' to append the next line to the pattern space.

Anyway, it's much easier if you match on a line that starts with a comma to decide whether or not to remove the newline, so that's what I did here:

sed 'N;/
,/s/"? *
//;P;D' title_csv

Input

$ cat title_csv
don't touch this line
don't touch this line either
This is a long abstract describing something. What follows is the tile for this sentence."
,Title1
seriously, don't touch this line
This is another sentence that is running on one line. On the next line you can find the title.
,Title2
also, don't touch this line

Output

$ sed 'N;/
,/s/"? *
//;P;D' title_csv
don't touch this line
don't touch this line either
This is a long abstract describing something. What follows is the tile for this sentence.,Title1
seriously, don't touch this line
This is another sentence that is running on one line. On the next line you can find the title.,Title2
also, don't touch this line