Between, before, after

Suppose we want to parse a string written in a simple computer language. We want to extract a substring between a start and end character.

With between() we can invoke index and rindex() on a string, and then take a substring in between. Some logic and testing is needed for correctness.

Method examples

Here we consider the 3 custom defs written in the Ruby language. We use substring syntax (which is like string slices) to return values.

def between(value, left, right)
    # Search from the start.
    pos1 = value.index(left)
    return "" if pos1 == nil
    pos1 += left.length
    
    # Search from the end.
    pos2 = value.rindex(right)
    return "" if pos2 == nil
    
    # Return a substring.
    return value[pos1, pos2 - pos1]
end    

def before(value, test)
    # Search from the start.
    pos1 = value.index(test)
    return "" if pos1 == nil
    return value[0, pos1]
end

def after(value, test)
    # Search from the end.
    pos1 = value.rindex(test)
    return "" if pos1 == nil
    pos1 += test.length
    return value[pos1..-1]
end


input = "DEFINE:A=TWO"

# Test between method.
puts between(input, "DEFINE:", "=")
puts between(input, ":", "=")

# Test before method.
puts before(input, ":")
puts before(input, "=")

# Test after method.
puts after(input, ":")
puts after(input, "DEFINE:")
puts after(input, "=")
A
A
DEFINE
DEFINE:A
A=TWO
A=TWO
TWO

Notes, results

When we examine the input string, we see that it contains syntax like a computer language. By calling between, before and after on this string, we test correctness.

To develop these 3 methods, we use index() and rindex() to search strings, and substring syntax to take parts of strings. We extract relative parts of strings.