While working on a tcl implementation of a cpp (i.e. C preprocessor) I found it necessary to splice together 
lines with escaped new lines while preserving the line number where the splice began.

Given a file of source lines that look like the following.

======
line(1)
line(2)\
line(3) with space after excape\    
line(4)\
line(5)
line(6)\
line(7)
line(8)
======

This code ...

======
set fid [open [file normalize "./filename"] r]
set text [read ${fid}]
close $fid
regsub -all -- {[\\][ \t]*[\n]} ${text} "\\\n" text
while { [regsub -all -- {[\\][\n](.*?[^\\])[\n]} ${text} "\\1\n\n" text] } {}
======

...produces the following results.

%|line|output|%
|1|`line(1)`|
|2|`line(2)line(3) with space after excapeline(4)line(5)`|
|3|| 
|4||
|5||
|6|`line(6)line(7)`|
|7||
|8|`line(8)`|

Notice that the lines have been spliced together and the spliced lines 
occur on the correct line in the output. Also notice that line 3, which
has white space after the backslash, was spliced. This is the way cpp
behaves when joining lines.

[Tom Krehbiel]

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