// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// backtrack is a regular expression search with submatch
// tracking for small regular expressions and texts. It allocates
// a bit vector with (length of input) * (length of prog) bits,
// to make sure it never explores the same (character position, instruction)
// state multiple times. This limits the search to run in time linear in
// the length of the test.
//
// backtrack is a fast replacement for the NFA code on small
// regexps when onepass cannot be used.
package regexp
import "regexp/syntax"
// A job is an entry on the backtracker's job stack. It holds
// the instruction pc and the position in the input.
type job struct {
pc uint32
arg int
pos int
}
const (
visitedBits = 32
maxBacktrackProg = 500 // len(prog.Inst) <= max
maxBacktrackVector = 256 * 1024 // bit vector size <= max (bits)
)
// bitState holds state for the backtracker.
type bitState struct {
prog *syntax.Prog
end int
cap []int
input input
jobs []job
visited []uint32
}
var notBacktrack *bitState = nil
// maxBitStateLen returns the maximum length of a string to search with
// the backtracker using prog.
func maxBitStateLen(prog *syntax.Prog) int {
if !shouldBacktrack(prog) {
return 0
}
return maxBacktrackVector / len(prog.Inst)
}
// newBitState returns a new bitState for the given prog,
// or notBacktrack if the size of the prog exceeds the maximum size that
// the backtracker will be run for.
func newBitState(prog *syntax.Prog) *bitState {
if !shouldBacktrack(prog) {
return notBacktrack
}
return &bitState{
prog: prog,
}
}
// shouldBacktrack reports whether the program is too
// long for the backtracker to run.
func shouldBacktrack(prog *syntax.Prog) bool {
return len(prog.Inst) <= maxBacktrackProg
}
// reset resets the state of the backtracker.
// end is the end position in the input.
// ncap is the number of captures.
func (b *bitState) reset(end int, ncap int) {
b.end = end
if cap(b.jobs) == 0 {
b.jobs = make([]job, 0, 256)
} else {
b.jobs = b.jobs[:0]
}
visitedSize := (len(b.prog.Inst)*(end+1) + visitedBits - 1) / visitedBits
if cap(b.visited) < visitedSize {
b.visited = make([]uint32, visitedSize, maxBacktrackVector/visitedBits)
} else {
b.visited = b.visited[:visitedSize]
for i := range b.visited {
b.visited[i] = 0
}
}
if cap(b.cap) < ncap {
b.cap = make([]int, ncap)
} else {
b.cap = b.cap[:ncap]
}
for i := range b.cap {
b.cap[i] = -1
}
}
// shouldVisit reports whether the combination of (pc, pos) has not
// been visited yet.
func (b *bitState) shouldVisit(pc uint32, pos int) bool {
n := uint(int(pc)*(b.end+1) + pos)
if b.visited[n/visitedBits]&(1<<(n&(visitedBits-1))) != 0 {
return false
}
b.visited[n/visitedBits] |= 1 << (n & (visitedBits - 1))
return true
}
// push pushes (pc, pos, arg) onto the job stack if it should be
// visited.
func (b *bitState) push(pc uint32, pos int, arg int) {
if b.prog.Inst[pc].Op == syntax.InstFail {
return
}
// Only check shouldVisit when arg == 0.
// When arg > 0, we are continuing a previous visit.
if arg == 0 && !b.shouldVisit(pc, pos) {
return
}
b.jobs = append(b.jobs, job{pc: pc, arg: arg, pos: pos})
}
// tryBacktrack runs a backtracking search starting at pos.
func (m *machine) tryBacktrack(b *bitState, i input, pc uint32, pos int) bool {
longest := m.re.longest
m.matched = false
b.push(pc, pos, 0)
for len(b.jobs) > 0 {
l := len(b.jobs) - 1
// Pop job off the stack.
pc := b.jobs[l].pc
pos := b.jobs[l].pos
arg := b.jobs[l].arg
b.jobs = b.jobs[:l]
// Optimization: rather than push and pop,
// code that is going to Push and continue
// the loop simply updates ip, p, and arg
// and jumps to CheckAndLoop. We have to
// do the ShouldVisit check that Push
// would have, but we avoid the stack
// manipulation.
goto Skip
CheckAndLoop:
if !b.shouldVisit(pc, pos) {
continue
}
Skip:
inst := b.prog.Inst[pc]
switch inst.Op {
default:
panic("bad inst")
case syntax.InstFail:
panic("unexpected InstFail")
case syntax.InstAlt:
// Cannot just
// b.push(inst.Out, pos, 0)
// b.push(inst.Arg, pos, 0)
// If during the processing of inst.Out, we encounter
// inst.Arg via another path, we want to process it then.
// Pushing it here will inhibit that. Instead, re-push
// inst with arg==1 as a reminder to push inst.Arg out
// later.
switch arg {
case 0:
b.push(pc, pos, 1)
pc = inst.Out
goto CheckAndLoop
case 1:
// Finished inst.Out; try inst.Arg.
arg = 0
pc = inst.Arg
goto CheckAndLoop
}
panic("bad arg in InstAlt")
case syntax.InstAltMatch:
// One opcode consumes runes; the other leads to match.
switch b.prog.Inst[inst.Out].Op {
case syntax.InstRune, syntax.InstRune1, syntax.InstRuneAny, syntax.InstRuneAnyNotNL:
// inst.Arg is the match.
b.push(inst.Arg, pos, 0)
pc = inst.Arg
pos = b.end
goto CheckAndLoop
}
// inst.Out is the match - non-greedy
b.push(inst.Out, b.end, 0)
pc = inst.Out
goto CheckAndLoop
case syntax.InstRune:
r, width := i.step(pos)
if !inst.MatchRune(r) {
continue
}
pos += width
pc = inst.Out
goto CheckAndLoop
case syntax.InstRune1:
r, width := i.step(pos)
if r != inst.Rune[0] {
continue
}
pos += width
pc = inst.Out
goto CheckAndLoop
case syntax.InstRuneAnyNotNL:
r, width := i.step(pos)
if r == '\n' || r == endOfText {
continue
}
pos += width
pc = inst.Out
goto CheckAndLoop
case syntax.InstRuneAny:
r, width := i.step(pos)
if r == endOfText {
continue
}
pos += width
pc = inst.Out
goto CheckAndLoop
case syntax.InstCapture:
switch arg {
case 0:
if 0 <= inst.Arg && inst.Arg < uint32(len(b.cap)) {
// Capture pos to register, but save old value.
b.push(pc, b.cap[inst.Arg], 1) // come back when we're done.
b.cap[inst.Arg] = pos
}
pc = inst.Out
goto CheckAndLoop
case 1:
// Finished inst.Out; restore the old value.
b.cap[inst.Arg] = pos
continue
}
panic("bad arg in InstCapture")
continue
case syntax.InstEmptyWidth:
if syntax.EmptyOp(inst.Arg)&^i.context(pos) != 0 {
continue
}
pc = inst.Out
goto CheckAndLoop
case syntax.InstNop:
pc = inst.Out
goto CheckAndLoop
case syntax.InstMatch:
// We found a match. If the caller doesn't care
// where the match is, no point going further.
if len(b.cap) == 0 {
m.matched = true
return m.matched
}
// Record best match so far.
// Only need to check end point, because this entire
// call is only considering one start position.
if len(b.cap) > 1 {
b.cap[1] = pos
}
if !m.matched || (longest && pos > 0 && pos > m.matchcap[1]) {
copy(m.matchcap, b.cap)
}
m.matched = true
// If going for first match, we're done.
if !longest {
return m.matched
}
// If we used the entire text, no longer match is possible.
if pos == b.end {
return m.matched
}
// Otherwise, continue on in hope of a longer match.
continue
}
panic("unreachable")
}
return m.matched
}
// backtrack runs a backtracking search of prog on the input starting at pos.
func (m *machine) backtrack(i input, pos int, end int, ncap int) bool {
if !i.canCheckPrefix() {
panic("backtrack called for a RuneReader")
}
startCond := m.re.cond
if startCond == ^syntax.EmptyOp(0) { // impossible
return false
}
if startCond&syntax.EmptyBeginText != 0 && pos != 0 {
// Anchored match, past beginning of text.
return false
}
b := m.b
b.reset(end, ncap)
m.matchcap = m.matchcap[:ncap]
for i := range m.matchcap {
m.matchcap[i] = -1
}
// Anchored search must start at the beginning of the input
if startCond&syntax.EmptyBeginText != 0 {
if len(b.cap) > 0 {
b.cap[0] = pos
}
return m.tryBacktrack(b, i, uint32(m.p.Start), pos)
}
// Unanchored search, starting from each possible text position.
// Notice that we have to try the empty string at the end of
// the text, so the loop condition is pos <= end, not pos < end.
// This looks like it's quadratic in the size of the text,
// but we are not clearing visited between calls to TrySearch,
// so no work is duplicated and it ends up still being linear.
width := -1
for ; pos <= end && width != 0; pos += width {
if len(m.re.prefix) > 0 {
// Match requires literal prefix; fast search for it.
advance := i.index(m.re, pos)
if advance < 0 {
return false
}
pos += advance
}
if len(b.cap) > 0 {
b.cap[0] = pos
}
if m.tryBacktrack(b, i, uint32(m.p.Start), pos) {
// Match must be leftmost; done.
return true
}
_, width = i.step(pos)
}
return false
}