egrep: empty (sub)expression : Unexpected behaviour for bash Loop on Mac
The shell expands the subexpression $file on the right hand side of the pipe before the variable has any value. So in both functions egrep is actually called with the empty string in place of $file. The first function doesn't throw an error because an empty pattern is a valid parameter for egrep both on macOS and on Linux. In the second fucntion the difference comes into play because the empty subexpression causes BSD egrep (the one used on macOS) to throw an error.

Example:

echo foo | egrep "|foo"
egrep: empty (sub)expression


as another example with $file having no value

echo foo | egrep "$file|foo"
egrep: empty (sub)expression


So, as a workaround in your function use:

egrep -e "$file" -e 'OK'


I have no idea why you are using egrep when you are looking for fixed strings.