The sub() function replaces the first occurrence of the pattern within the source string.

result = sub (pattern, replacement, sourcestring);
pattern Required. The regular expression pattern to search for.

Required. The replacement string.

sourcestring Required. The source string to search for the first occurrence of pattern.

The resulting string

newstring = sub("\n$", "", textstring)

In this example, the first occurrence of a trailing new line is replaced with nothing, effectively chopping it off.

For more information, please see gsub.