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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Simplify returns a regexp equivalent to re but without counted repetitions
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// and with various other simplifications, such as rewriting /(?:a+)+/ to /a+/.
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// The resulting regexp will execute correctly but its string representation
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// will not produce the same parse tree, because capturing parentheses
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// may have been duplicated or removed. For example, the simplified form
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// for /(x){1,2}/ is /(x)(x)?/ but both parentheses capture as $1.
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// The returned regexp may share structure with or be the original.
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func (re *Regexp) Simplify() *Regexp {
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case OpCapture, OpConcat, OpAlternate:
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// Simplify children, building new Regexp if children change.
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for i, sub := range re.Sub {
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nsub := sub.Simplify()
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if nre == re && nsub != sub {
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nre.Sub = append(nre.Sub0[:0], re.Sub[:i]...)
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nre.Sub = append(nre.Sub, nsub)
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case OpStar, OpPlus, OpQuest:
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sub := re.Sub[0].Simplify()
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return simplify1(re.Op, re.Flags, sub, re)
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// Special special case: x{0} matches the empty string
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// and doesn't even need to consider x.
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if re.Min == 0 && re.Max == 0 {
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return &Regexp{Op: OpEmptyMatch}
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sub := re.Sub[0].Simplify()
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// x{n,} means at least n matches of x.
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// Special case: x{0,} is x*.
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return simplify1(OpStar, re.Flags, sub, nil)
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// Special case: x{1,} is x+.
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return simplify1(OpPlus, re.Flags, sub, nil)
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// General case: x{4,} is xxxx+.
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nre := &Regexp{Op: OpConcat}
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nre.Sub = nre.Sub0[:0]
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for i := 0; i < re.Min-1; i++ {
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nre.Sub = append(nre.Sub, sub)
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nre.Sub = append(nre.Sub, simplify1(OpPlus, re.Flags, sub, nil))
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// Special case x{0} handled above.
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// Special case: x{1} is just x.
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if re.Min == 1 && re.Max == 1 {
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// General case: x{n,m} means n copies of x and m copies of x?
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// The machine will do less work if we nest the final m copies,
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// so that x{2,5} = xx(x(x(x)?)?)?
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// Build leading prefix: xx.
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prefix = &Regexp{Op: OpConcat}
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prefix.Sub = prefix.Sub0[:0]
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for i := 0; i < re.Min; i++ {
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prefix.Sub = append(prefix.Sub, sub)
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// Build and attach suffix: (x(x(x)?)?)?
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suffix := simplify1(OpQuest, re.Flags, sub, nil)
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for i := re.Min + 1; i < re.Max; i++ {
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nre2 := &Regexp{Op: OpConcat}
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nre2.Sub = append(nre2.Sub0[:0], sub, suffix)
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suffix = simplify1(OpQuest, re.Flags, nre2, nil)
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prefix.Sub = append(prefix.Sub, suffix)
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// Some degenerate case like min > max or min < max < 0.
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// Handle as impossible match.
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return &Regexp{Op: OpNoMatch}
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// simplify1 implements Simplify for the unary OpStar,
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// OpPlus, and OpQuest operators. It returns the simple regexp
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// Regexp{Op: op, Flags: flags, Sub: {sub}}
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// under the assumption that sub is already simple, and
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// without first allocating that structure. If the regexp
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// to be returned turns out to be equivalent to re, simplify1
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// returns re instead.
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// simplify1 is factored out of Simplify because the implementation
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// for other operators generates these unary expressions.
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// Letting them call simplify1 makes sure the expressions they
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// generate are simple.
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func simplify1(op Op, flags Flags, sub, re *Regexp) *Regexp {
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// Special case: repeat the empty string as much as
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// you want, but it's still the empty string.
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if sub.Op == OpEmptyMatch {
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// The operators are idempotent if the flags match.
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if op == sub.Op && flags&NonGreedy == sub.Flags&NonGreedy {
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if re != nil && re.Op == op && re.Flags&NonGreedy == flags&NonGreedy && sub == re.Sub[0] {
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re = &Regexp{Op: op, Flags: flags}
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re.Sub = append(re.Sub0[:0], sub)
added
removed
src/pkg/exp/regexp/syntax/simplify.go
