Regex: Implement lookarounds for fixed literal strings

We do not support anything else than a plain literal string for
lookarounds.

src/regex_impl.cc

@@ -29,6 +29,10 @@ struct ParsedRegex
         SubjectBegin,
         SubjectEnd,
         ResetStart,
+        LookAhead,
+        LookBehind,
+        NegativeLookAhead,
+        NegativeLookBehind,
     };
 
     struct Quantifier
@@ -116,9 +120,9 @@ private:
         return res;
     }
 
-    AstNodePtr alternative()
+    AstNodePtr alternative(ParsedRegex::Op op = ParsedRegex::Sequence)
     {
-        AstNodePtr res = new_node(ParsedRegex::Sequence);
+        AstNodePtr res = new_node(op);
         while (auto node = term())
             res->children.push_back(std::move(node));
         if (res->children.empty())
@@ -175,8 +179,44 @@ private:
             case '.': ++m_pos; return new_node(ParsedRegex::AnyChar);
             case '(':
             {
-                ++m_pos;
-                auto content = disjunction(m_parsed_regex.capture_count++);
+                auto advance = [&]() {
+                    if (++m_pos == m_regex.end())
+                        parse_error("unclosed parenthesis");
+                    return *m_pos;
+                };
+
+                AstNodePtr content;
+                if (advance() == '?')
+                {
+                    auto c = advance();
+                    if (c == ':')
+                        content = disjunction(-1);
+                    else if (contains("=!<", c))
+                    {
+                        bool behind = false;
+                        if (c == '<')
+                        {
+                            advance();
+                            behind = true;
+                        }
+
+                        auto type = *m_pos++;
+                        if (type == '=')
+                            content = alternative(behind ? ParsedRegex::LookBehind
+                                                         : ParsedRegex::LookAhead);
+                        else if (type == '!')
+                            content = alternative(behind ? ParsedRegex::NegativeLookBehind
+                                                         : ParsedRegex::NegativeLookAhead);
+                        else
+                            parse_error("invalid disjunction");
+
+                         validate_lookaround(content);
+                    }
+                    else
+                        parse_error("invalid disjunction");
+                }
+                else
+                    content = disjunction(m_parsed_regex.capture_count++);
 
                 if (at_end() or *m_pos != ')')
                     parse_error("unclosed parenthesis");
@@ -245,7 +285,7 @@ private:
 
         if (contains("^$\\.*+?()[]{}|", cp)) // SyntaxCharacter
             return new_node(ParsedRegex::Literal, cp);
-        parse_error("unknown atom escape");
+        parse_error(format("unknown atom escape '{}'", cp));
     }
 
     AstNodePtr character_class()
@@ -395,6 +435,13 @@ private:
                                    StringView{m_pos.base(), m_regex.end()}));
     }
 
+    void validate_lookaround(const AstNodePtr& node)
+    {
+        for (auto& child : node->children)
+            if (child->op != ParsedRegex::Literal)
+                parse_error("Lookaround can only contain literals");
+    }
+
     ParsedRegex m_parsed_regex;
     StringView m_regex;
     Iterator m_pos;
@@ -406,6 +453,11 @@ private:
         bool neg;
     };
 
+    StringView peek(ByteCount count)
+    {
+        return StringView{m_pos.base(), m_regex.end()}.substr(0, count);
+    }
+
     static const CharacterClassEscape character_class_escapes[8];
 };
 
@@ -439,6 +491,10 @@ struct CompiledRegex
         NotWordBoundary,
         SubjectBegin,
         SubjectEnd,
+        LookAhead,
+        LookBehind,
+        NegativeLookAhead,
+        NegativeLookBehind,
     };
 
     using Offset = unsigned;
@@ -516,6 +572,22 @@ private:
 
                 break;
             }
+            case ParsedRegex::LookAhead:
+                push_op(CompiledRegex::LookAhead);
+                push_string(node->children);
+                break;
+            case ParsedRegex::LookBehind:
+                push_op(CompiledRegex::LookBehind);
+                push_string(node->children, true);
+                break;
+            case ParsedRegex::NegativeLookAhead:
+                push_op(CompiledRegex::NegativeLookAhead);
+                push_string(node->children);
+                break;
+            case ParsedRegex::NegativeLookBehind:
+                push_op(CompiledRegex::NegativeLookBehind);
+                push_string(node->children, true);
+                break;
             case ParsedRegex::LineStart:
                 push_op(CompiledRegex::LineStart);
                 break;
@@ -631,6 +703,20 @@ private:
         utf8::dump(std::back_inserter(m_program.bytecode), cp);
     }
 
+    void push_string(const Vector<ParsedRegex::AstNodePtr>& codepoints, bool reversed = false)
+    {
+        if (codepoints.size() > 127)
+            throw runtime_error{"Too long literal string"};
+
+        push_byte(codepoints.size());
+        if (reversed)
+            for (auto& cp : codepoints | reverse()) 
+                push_codepoint(cp->value);
+        else
+            for (auto& cp : codepoints) 
+                push_codepoint(cp->value);
+    }
+
     CompiledRegex m_program;
     const ParsedRegex& m_parsed_regex;
 };
@@ -687,6 +773,27 @@ void dump_regex(const CompiledRegex& program)
             case CompiledRegex::SubjectEnd:
                 printf("subject end\n");
                 break;
+            case CompiledRegex::LookAhead:
+            case CompiledRegex::NegativeLookAhead:
+            case CompiledRegex::LookBehind:
+            case CompiledRegex::NegativeLookBehind:
+            {
+                int count = *pos++;
+                StringView str{pos, pos + count};
+                const char* name = nullptr;
+                if (op == CompiledRegex::LookAhead)
+                    name = "look ahead";
+                if (op == CompiledRegex::NegativeLookAhead)
+                    name = "negative look ahead";
+                if (op == CompiledRegex::LookBehind)
+                    name = "look behind";
+                if (op == CompiledRegex::NegativeLookBehind)
+                    name = "negative look behind";
+
+                printf("%s (%s)\n", name, (const char*)str.zstr());
+                pos += count;
+                break;
+            }
             case CompiledRegex::Match:
                 printf("match\n");
         }
@@ -783,6 +890,32 @@ struct ThreadedRegexVM
                     if (m_pos != m_end)
                         return StepResult::Failed;
                     break;
+                case CompiledRegex::LookAhead:
+                case CompiledRegex::NegativeLookAhead:
+                {
+                    int count = *thread.inst++;
+                    for (auto it = m_pos; count and it != m_end; ++it, --count)
+                        if (*it != utf8::read(thread.inst))
+                            break;
+                    if ((op == CompiledRegex::LookAhead and count != 0) or
+                        (op == CompiledRegex::NegativeLookAhead and count == 0))
+                        return StepResult::Failed;
+                    thread.inst = utf8::advance(thread.inst, prog_end, CharCount{count - 1});
+                    break;
+                }
+                case CompiledRegex::LookBehind:
+                case CompiledRegex::NegativeLookBehind:
+                {
+                    int count = *thread.inst++;
+                    for (auto it = m_pos-1; count and it >= m_begin; --it, --count)
+                        if (*it != utf8::read(thread.inst))
+                            break;
+                    if ((op == CompiledRegex::LookBehind and count != 0) or
+                        (op == CompiledRegex::NegativeLookBehind and count == 0))
+                        return StepResult::Failed;
+                    thread.inst = utf8::advance(thread.inst, prog_end, CharCount{count - 1});
+                    break;
+                }
                 case CompiledRegex::Match:
                     thread.inst = nullptr;
                     return StepResult::Matched;
@@ -823,7 +956,7 @@ struct ThreadedRegexVM
             m_threads.erase(std::remove_if(m_threads.begin(), m_threads.end(),
                                            [](const Thread& t) { return t.inst == nullptr; }), m_threads.end());
             if (m_threads.empty())
-                return false;
+                return found_match;
         }
 
         // Step remaining threads to see if they match without consuming anything else
@@ -1034,6 +1167,30 @@ auto test_regex = UnitTest{[]{
         kak_assert(vm.exec("foooo", true, true));
         kak_assert(StringView{vm.m_captures[2], vm.m_captures[3]} == "fo");
     }
+
+    {
+        TestVM vm{R"((?=foo).)"};
+        kak_assert(vm.exec("barfoo", false, true));
+        kak_assert(StringView{vm.m_captures[0], vm.m_captures[1]} == "f");
+    }
+
+    {
+        TestVM vm{R"((?!foo)...)"};
+        kak_assert(not vm.exec("foo"));
+        kak_assert(vm.exec("qux"));
+    }
+
+    {
+        TestVM vm{R"(...(?<=foo))"};
+        kak_assert(vm.exec("foo"));
+        kak_assert(not vm.exec("qux"));
+    }
+
+    {
+        TestVM vm{R"(...(?<!foo))"};
+        kak_assert(not vm.exec("foo"));
+        kak_assert(vm.exec("qux"));
+    }
 }};
 
 }