Question

Having to use regex for the first time and although I almost achieved what I require I do not seems to be able to combine into a single statement.

I have a string of words where I wish to replace \n if it is not preceded by a dot OR not preceded by a dot space.

I can run either of these two statements to achieve the required result. However, if I either run them one after another or try to combine them into a single regex, it does not work.

//replaces \n if not preceded by dot space
xx = xx.replaceAll("(.+)(?<!\\. )\n", "$1 ");


//replaces \n if not preceded by dot
xx = xx.replaceAll("(.+)(?<!\\.)\n", "$1 "); 


//one of my attempts to combine into a single statement
xx = xx.replaceAll("(.+)(?<!\\. )\n|(?<!\\.)\n", "$1 ");

Example of String I’m trying to fix.

BEFORE

This is some text which may\n
have a newline character to break the line\n
but I only want to remove it if it's not preceded with a full.\n
or it's not preceded with a full stop and a space. \n

AFTER

This is some text which may
have a newline character to break the line 
but I only want to remove it if it's not preceded with a full.\n
or it's not preceded with a full stop and a space. \n

I think I’m close, but being new to regex, I am getting more confused the more I read.

Answer 1

It’s easier than you think:

String resultString = subjectString.replaceAll("(?<!\\. ?)\n", " ");

Explanation:

(?<!   # Assert that the previous characters are not...
 \.    # a dot
 [ ]?  # optionally followed by a space
)      # End of lookbehind
\n     # Match a newline character

So you don’t need to match (.+) in the first place, only to replace it with itself afterwards. Incidentally, here’s what tripped you up:

(.+)(?<!\. )\n|(?<!\.)\n

is logically grouped as

(.+)(?<!\. )\n   # Match this
|                # or
(?<!\.)\n        # this

so the (.+) is only matched if there is no space after the dot.