Abstract

Symbolic pattern matching as offered by Lisp dialects allows to scan Sexpressions, to verify their shape and to extract or compare subparts of them. A rich set of patterns exists and among them alternate patterns. Such patterns are handled through backtrack: a failure forces the pattern matcher to regress to its last point of choice and to try another branch. The usual naive backtrack algorithm forgets all the informations acquired on the datum from the last point of choice up to the failure point. Our algorithm provides intelligent backtrack i.e. these informations are given back to the pattern matcher which can then use them to choose the appropriate backtrack point therefore eliminating redundant tests or dead-end branches. It turns out that this very simple idea "Don't throw any information painfully acquired", forms the basis of many clever algorithms such as Knuth-Morris-Pratt (KMP) or Boyer-Moore (BM) for strings, Hoffmann-O'Donnell for trees etc. The other benefit comes from th...

Citations