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 code of 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.^{172Behaviour that is fully compatible would require unlimited look-ahead.} Below are some examples:

Read a line:

read_string(Input, "\n", "\r", Sep, String)

Read a line, stripping leading and trailing white space:

read_string(Input, "\n", "\r\t ", Sep, String)

Read up to‘,’or‘)’, unifying Sep with 0', i.e. Unicode 44, or 0'), i.e. Unicode 41:

read_string(Input, ",)", "\t ", Sep, String)