To understand why this is useful, you must understand the problem. Consider the common task of searching for whole words in a string. The pattern "%Wdog%W" (. the word surrounded by non-word characters) works in most cases, but fails at the start and beginning of strings, where the respective 'guards' do not match. The frontier pattern %f[c] solves the problem: "%f[%w]dog%f[%W]". The first guard says 'match when %w first becomes true' and the second says 'match when %W becomes true', and they both match appropriately at the edges of the strings.