  This paper introduces to the calculus of regular expressions a replace
operator, ->, and defines a set of replacement expressions that concisely
encode several alternate variations of the operation.
  The basic case is unconditional obligatory replacement:
  UPPER -> LOWER
  Conditional versions of replacement, such as,
  UPPER -> LOWER || LEFT _ RIGHT constrain the operation by left and right
contexts. UPPER, LOWER, LEFT, and RIGHT may be regular expressions of any
complexity.
  Replace expressions denote regular relations. The replace operator is defined
in terms of other regular expression operators using techniques introduced by
Ronald M. Kaplan and Martin Kay in "Regular Models of Phonological Rule
Systems" (Computational Linguistics 20:3 331-378. 1994).
