1. Skip all characters that match PadChars
2. Read up to a character that matches SepChars or end of file
3. Discard trailing characters that match PadChars from the collected input
4. Unify String with a string created from the input and Sep with the separator character read. If input was terminated by the end of the input, Sep is unified with -1.

The predicate read_string/5 called repeatedly on an input until Sep is -1 (end of file) is equivalent to reading the entire file into a string and calling split_string/4, provided that SepChars and PadChars are not partially overlapping.^{150Behaviour that is fully compatible would requite unlimited look-ahead.} Below are some examples:

% Read a line
read_string(Input, "\n", "\r", End, String)
% Read a line, stripping leading and trailing white space
read_string(Input, "\n", "\r\t ", End, String)
% Read upto , or ), unifying End with 0', or 0')
read_string(Input, ",)", "\t ", End, String)