Add support for Hamming distance approx matching

doc/dev-reference/compilation.rst

@@ -287,6 +287,7 @@ which provides the following fields:
 * ``min_length``: The minimum match length (from start to end) required to
   successfully match this expression.
 * ``edit_distance``: Match this expression within a given Levenshtein distance.
+* ``hamming_distance``: Match this expression within a given Hamming distance.
 
 These parameters either allow the set of matches produced by a pattern to be
 constrained at compile time (rather than relying on the application to process
@@ -299,10 +300,15 @@ and a ``max_offset`` of 15 will not produce matches when scanned against
 streams ``foo0123bar`` or ``foo0123456bar``.
 
 Similarly, the pattern :regexp:`/foobar/` when given an ``edit_distance`` of 2
-will produce matches when scanned against ``foobar``, ``fooba``, ``fobr``,
+will produce matches when scanned against ``foobar``, ``f00bar``, ``fooba``,
-``fo_baz``, ``foooobar``, and anything else that lies within edit distance of 2
+``fobr``, ``fo_baz``, ``foooobar``, and anything else that lies within edit
-(as defined by Levenshtein distance). For more details, see the
+distance of 2 (as defined by Levenshtein distance).
-:ref:`approximate_matching` section.
+
+When the same pattern :regexp:`/foobar/` is given a ``hamming_distance`` of 2,
+it will produce matches when scanned against ``foobar``, ``boofar``,
+``f00bar``, and anything else with at most two characters substituted from the
+original pattern. For more details, see the :ref:`approximate_matching`
+section.
 
 =================
 Prefiltering Mode
@@ -398,13 +404,20 @@ follows:
 
 #. **Edit distance** is defined as Levenshtein distance. That is, there are
    three possible edit types considered: insertion, removal and substitution.
-   More formal description can be found on
+   A more formal description can be found on
    `Wikipedia <https://en.wikipedia.org/wiki/Levenshtein_distance>`_.
 
-#. **Approximate matching** will match all *corpora* within a given edit
+#. **Hamming distance** is the number of positions by which two strings of
-   distance. That is, given a pattern, approximate matching will match anything
+   equal length differ. That is, it is the number of substitutions required to
-   that can be edited to arrive at a corpus that exactly matches the original
+   convert one string to the other. There are no insertions or removals when
-   pattern.
+   approximate matching using a Hamming distance. A more formal description can
+   be found on
+   `Wikipedia <https://en.wikipedia.org/wiki/Hamming_distance>`_.
+
+#. **Approximate matching** will match all *corpora* within a given edit or
+   Hamming distance. That is, given a pattern, approximate matching will match
+   anything that can be edited to arrive at a corpus that exactly matches the
+   original pattern.
 
 #. **Matching semantics** are exactly the same as described in :ref:`semantics`.
 
@@ -437,7 +450,9 @@ matching support. Here they are, in a nutshell:
     reduce to so-called "vacuous" patterns (patterns that match everything). For
     example, pattern :regexp:`/foo/` with edit distance 3, if implemented,
     would reduce to matching zero-length buffers. Such patterns will result in a
-    "Pattern cannot be approximately matched" compile error.
+    "Pattern cannot be approximately matched" compile error. Approximate
+    matching within a Hamming distance does not remove symbols, so will not
+    reduce to a vacuous pattern.
   * Finally, due to the inherent complexities of defining matching behavior,
     approximate matching implements a reduced subset of regular expression
     syntax. Approximate matching does not support UTF-8 (and other

src/compiler/compiler.cpp

@@ -78,7 +78,8 @@ void validateExt(const hs_expr_ext &ext) {
     static const unsigned long long ALL_EXT_FLAGS = HS_EXT_FLAG_MIN_OFFSET |
                                                     HS_EXT_FLAG_MAX_OFFSET |
                                                     HS_EXT_FLAG_MIN_LENGTH |
-                                                    HS_EXT_FLAG_EDIT_DISTANCE;
+                                                    HS_EXT_FLAG_EDIT_DISTANCE |
+                                                    HS_EXT_FLAG_HAMMING_DISTANCE;
     if (ext.flags & ~ALL_EXT_FLAGS) {
         throw CompileError("Invalid hs_expr_ext flag set.");
     }
@@ -96,6 +97,13 @@ void validateExt(const hs_expr_ext &ext) {
         throw CompileError("In hs_expr_ext, min_length must be less than or "
                            "equal to max_offset.");
     }
+
+    if ((ext.flags & HS_EXT_FLAG_EDIT_DISTANCE) &&
+        (ext.flags & HS_EXT_FLAG_HAMMING_DISTANCE)) {
+        throw CompileError("In hs_expr_ext, cannot have both edit distance and "
+                           "Hamming distance.");
+    }
+
 }
 
 ParsedExpression::ParsedExpression(unsigned index_in, const char *expression,
@@ -103,7 +111,7 @@ ParsedExpression::ParsedExpression(unsigned index_in, const char *expression,
                                    const hs_expr_ext *ext)
     : expr(index_in, flags & HS_FLAG_ALLOWEMPTY, flags & HS_FLAG_SINGLEMATCH,
            false, flags & HS_FLAG_PREFILTER, SOM_NONE, report, 0, MAX_OFFSET,
-           0, 0) {
+           0, 0, 0) {
     ParseMode mode(flags);
 
     component = parse(expression, mode);
@@ -158,6 +166,9 @@ ParsedExpression::ParsedExpression(unsigned index_in, const char *expression,
         if (ext->flags & HS_EXT_FLAG_EDIT_DISTANCE) {
             expr.edit_distance = ext->edit_distance;
         }
+        if (ext->flags & HS_EXT_FLAG_HAMMING_DISTANCE) {
+            expr.hamm_distance = ext->hamming_distance;
+        }
     }
 
     // These are validated in validateExt, so an error will already have been

src/compiler/expression_info.h

@@ -45,11 +45,13 @@ public:
     ExpressionInfo(unsigned int index_in, bool allow_vacuous_in,
                    bool highlander_in, bool utf8_in, bool prefilter_in,
                    som_type som_in, ReportID report_in, u64a min_offset_in,
-                   u64a max_offset_in, u64a min_length_in, u32 edit_distance_in)
+                   u64a max_offset_in, u64a min_length_in, u32 edit_distance_in,
+                   u32 hamm_distance_in)
         : index(index_in), report(report_in), allow_vacuous(allow_vacuous_in),
           highlander(highlander_in), utf8(utf8_in), prefilter(prefilter_in),
           som(som_in), min_offset(min_offset_in), max_offset(max_offset_in),
-          min_length(min_length_in), edit_distance(edit_distance_in) {}
+          min_length(min_length_in), edit_distance(edit_distance_in),
+          hamm_distance(hamm_distance_in) {}
 
     /**
      * \brief Index of the expression represented by this graph.
@@ -95,6 +97,7 @@ public:
      * 0 if not used.
      */
     u32 edit_distance;
+    u32 hamm_distance;
 };
 
 }

src/hs_compile.h

@@ -258,6 +258,13 @@ typedef struct hs_expr_ext {
      * hs_expr_ext::flags field.
      */
     unsigned edit_distance;
+
+    /**
+     * Allow patterns to approximately match within this Hamming distance. To
+     * use this parameter, set the @ref HS_EXT_FLAG_HAMMING_DISTANCE flag in the
+     * hs_expr_ext::flags field.
+     */
+    unsigned hamming_distance;
 } hs_expr_ext_t;
 
 /**
@@ -281,6 +288,9 @@ typedef struct hs_expr_ext {
 /** Flag indicating that the hs_expr_ext::edit_distance field is used. */
 #define HS_EXT_FLAG_EDIT_DISTANCE   8ULL
 
+/** Flag indicating that the hs_expr_ext::hamming_distance field is used. */
+#define HS_EXT_FLAG_HAMMING_DISTANCE 16ULL
+
 /** @} */
 
 /**

src/nfagraph/ng.cpp

@@ -347,14 +347,19 @@ bool NG::addGraph(ExpressionInfo &expr, unique_ptr<NGHolder> g_ptr) {
         throw CompileError(expr.index, "Pattern can never match.");
     }
 
+    bool hamming = expr.hamm_distance > 0;
+    u32 e_dist = hamming ? expr.hamm_distance : expr.edit_distance;
+
+    DEBUG_PRINTF("edit distance = %u hamming = %s\n", e_dist, hamming ? "true" : "false");
+
     // validate graph's suitability for fuzzing before resolving asserts
-    validate_fuzzy_compile(g, expr.edit_distance, expr.utf8, cc.grey);
+    validate_fuzzy_compile(g, e_dist, hamming, expr.utf8, cc.grey);
 
     resolveAsserts(rm, g, expr);
     dumpDotWrapper(g, expr, "02_post_assert_resolve", cc.grey);
     assert(allMatchStatesHaveReports(g));
 
-    make_fuzzy(g, expr.edit_distance, cc.grey);
+    make_fuzzy(g, e_dist, hamming, cc.grey);
     dumpDotWrapper(g, expr, "02a_post_fuzz", cc.grey);
 
     pruneUseless(g);

src/nfagraph/ng_expr_info.cpp

@@ -161,14 +161,17 @@ void fillExpressionInfo(ReportManager &rm, const CompileContext &cc,
         throw CompileError(expr.index, "Pattern can never match.");
     }
 
+    bool hamming = expr.hamm_distance > 0;
+    u32 e_dist = hamming ? expr.hamm_distance : expr.edit_distance;
+
     // validate graph's suitability for fuzzing
-    validate_fuzzy_compile(g, expr.edit_distance, expr.utf8, cc.grey);
+    validate_fuzzy_compile(g, e_dist, hamming, expr.utf8, cc.grey);
 
     resolveAsserts(rm, g, expr);
     assert(allMatchStatesHaveReports(g));
 
     // fuzz graph - this must happen before any transformations are made
-    make_fuzzy(g, expr.edit_distance, cc.grey);
+    make_fuzzy(g, e_dist, hamming, cc.grey);
 
     pruneUseless(g);
     pruneEmptyVertices(g);

src/nfagraph/ng_fuzzy.cpp

@@ -144,6 +144,7 @@ vector<flat_set<NFAVertex>> gatherPredecessorsByDepth(const NGHolder &g,
 struct ShadowGraph {
     NGHolder &g;
     u32 edit_distance;
+    bool hamming;
     map<pair<NFAVertex, u32>, NFAVertex> shadow_map;
     map<pair<NFAVertex, u32>, NFAVertex> helper_map;
     map<NFAVertex, NFAVertex> clones;
@@ -151,13 +152,17 @@ struct ShadowGraph {
     vector<pair<NFAVertex, NFAVertex>> edges_to_be_added;
     flat_set<NFAVertex> orig;
 
-    ShadowGraph(NGHolder &g_in, u32 ed_in) : g(g_in), edit_distance(ed_in) {}
+    ShadowGraph(NGHolder &g_in, u32 ed_in, bool hamm_in)
+        : g(g_in), edit_distance(ed_in), hamming(hamm_in) {}
 
     void fuzz_graph() {
         if (edit_distance == 0) {
             return;
         }
 
+        DEBUG_PRINTF("edit distance = %u hamming = %s\n", edit_distance,
+                     hamming ? "true" : "false");
+
         // step 1: prepare the vertices, helpers and shadows according to
         // the original graph
         prepare_graph();
@@ -167,7 +172,9 @@ struct ShadowGraph {
 
         // step 3: set up reports for newly created vertices (and make clones
         // if necessary)
+        if (!hamming) {
             create_reports();
+        }
 
         // step 4: wire up shadow graph and helpers for insert/replace/remove
         connect_shadow_graph();
@@ -244,6 +251,16 @@ private:
 
                 // if there's nowhere to go from this vertex, no helper needed
                 if (proper_out_degree(v, g) < 1) {
+                    DEBUG_PRINTF("No helper for node ID: %zu (level %u)\n",
+                                 g[shadow_v].index, dist);
+                    helper_map[make_pair(v, dist)] = shadow_v;
+                    continue;
+                }
+
+                // start and startDs only have helpers for insert, so not Hamming
+                if (hamming && is_any_start(v, g)) {
+                    DEBUG_PRINTF("No helper for node ID: %zu (level %u)\n",
+                                 g[shadow_v].index, dist);
                     helper_map[make_pair(v, dist)] = shadow_v;
                     continue;
                 }
@@ -256,6 +273,8 @@ private:
                 g[helper_v].char_reach = CharReach::dot();
                 // do not copy virtual start's assert flags
                 if (is_virtual_start(v, g)) {
+                    DEBUG_PRINTF("Helper node ID is virtual start: %zu (level %u)\n",
+                             g[helper_v].index, dist);
                     g[helper_v].assert_flags = 0;
                 }
                 helper_map[make_pair(v, dist)] = helper_v;
@@ -272,7 +291,7 @@ private:
         const auto &cur_shadow_helper = helper_map[make_pair(v, dist)];
 
         // multiple insert
-        if (dist > 1) {
+        if (!hamming && dist > 1) {
             const auto &prev_level_helper = helper_map[make_pair(v, dist - 1)];
             connect_to_clones(prev_level_helper, cur_shadow_helper);
         }
@@ -429,15 +448,17 @@ private:
                 connect_preds(v, dist);
 
                 // handle helpers
-                if (dist > 0) {
+                if (!hamming && dist > 0) {
                     connect_helpers(v, dist);
                 }
             }
 
             // handle removals
+            if (!hamming) {
                 connect_removals(v);
             }
         }
+    }
 
     void connect_to_targets(NFAVertex src, const flat_set<NFAVertex> &targets) {
         for (auto dst : targets) {
@@ -636,8 +657,8 @@ bool will_turn_vacuous(const NGHolder &g, u32 edit_distance) {
     return false;
 }
 
-void validate_fuzzy_compile(const NGHolder &g, u32 edit_distance, bool utf8,
+void validate_fuzzy_compile(const NGHolder &g, u32 edit_distance, bool hamming,
-                            const Grey &grey) {
+                            bool utf8, const Grey &grey) {
     if (edit_distance == 0) {
         return;
     }
@@ -657,13 +678,14 @@ void validate_fuzzy_compile(const NGHolder &g, u32 edit_distance, bool utf8,
                                "approximate matching.");
         }
     }
-    if (will_turn_vacuous(g, edit_distance)) {
+    if (!hamming && will_turn_vacuous(g, edit_distance)) {
         throw CompileError("Approximate matching patterns that reduce to "
                            "vacuous patterns are disallowed.");
     }
 }
 
-void make_fuzzy(NGHolder &g, u32 edit_distance, const Grey &grey) {
+void make_fuzzy(NGHolder &g, u32 edit_distance, bool hamming,
+                const Grey &grey) {
     if (edit_distance == 0) {
         return;
     }
@@ -671,7 +693,7 @@ void make_fuzzy(NGHolder &g, u32 edit_distance, const Grey &grey) {
     assert(grey.allowApproximateMatching);
     assert(grey.maxEditDistance >= edit_distance);
 
-    ShadowGraph sg(g, edit_distance);
+    ShadowGraph sg(g, edit_distance, hamming);
     sg.fuzz_graph();
 
     // For safety, enforce limit on actual vertex count.

src/nfagraph/ng_fuzzy.h

@@ -40,10 +40,10 @@ struct Grey;
 class NGHolder;
 class ReportManager;
 
-void validate_fuzzy_compile(const NGHolder &g, u32 edit_distance, bool utf8,
+void validate_fuzzy_compile(const NGHolder &g, u32 edit_distance, bool hamming,
-                            const Grey &grey);
+                            bool utf8, const Grey &grey);
 
-void make_fuzzy(NGHolder &g, u32 edit_distance, const Grey &grey);
+void make_fuzzy(NGHolder &g, u32 edit_distance, bool hamming, const Grey &grey);
 }
 
 #endif // NG_FUZZY_H

src/parser/shortcut_literal.cpp

@@ -170,7 +170,7 @@ bool shortcutLiteral(NG &ng, const ParsedExpression &pe) {
 
     // XXX: don't shortcut literals with extended params (yet)
     if (expr.min_offset || expr.max_offset != MAX_OFFSET || expr.min_length ||
-        expr.edit_distance) {
+        expr.edit_distance || expr.hamm_distance) {
         DEBUG_PRINTF("extended params not allowed\n");
         return false;
     }

unit/hyperscan/bad_patterns.txt

@@ -144,3 +144,4 @@
 147:/\b\BMYBt/s{edit_distance=1} #Pattern can never match.
 148:/\QÀ\Eaaaa/8 #Expression is not valid UTF-8.
 149:/[\QÀ\Eaaaa]/8 #Expression is not valid UTF-8.
+150:/abcd/{edit_distance=1,hamming_distance=1} #In hs_expr_ext, cannot have both edit distance and Hamming distance.

unit/hyperscan/expr_info.cpp

@@ -84,6 +84,13 @@ ostream& operator<<(ostream &os, const hs_expr_ext &ext) {
         os << "edit_distance=" << ext.edit_distance;
         first = false;
     }
+    if (ext.flags & HS_EXT_FLAG_HAMMING_DISTANCE) {
+        if (!first) {
+            os << ", ";
+        }
+        os << "hamming_distance=" << ext.hamming_distance;
+        first = false;
+    }
     return os;
 }
 
@@ -171,7 +178,7 @@ TEST_P(ExprInfop, check_ext_null) {
     free(info);
 }
 
-static const hs_expr_ext NO_EXT_PARAM = { 0, 0, 0, 0, 0 };
+static const hs_expr_ext NO_EXT_PARAM = { 0, 0, 0, 0, 0, 0 };
 
 static const expected_info ei_test[] = {
     {"abc", NO_EXT_PARAM, 3, 3, 0, 0, 0},
@@ -214,38 +221,68 @@ static const expected_info ei_test[] = {
     {"(foo|bar)\\z", NO_EXT_PARAM, 3, 3, 0, 1, 1},
 
     // Some cases with extended parameters.
-    {"^abc.*def", {HS_EXT_FLAG_MAX_OFFSET, 0, 10, 0, 0}, 6, 10, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_MAX_OFFSET, 0, 10, 0, 0, 0}, 6, 10, 0, 0, 0},
-    {"^abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 100, 0}, 100, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 100, 0, 0}, 100, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_MAX_OFFSET, 0, 10, 0, 0}, 6, 10, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_MAX_OFFSET, 0, 10, 0, 0, 0}, 6, 10, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 100, 0}, 100, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 100, 0, 0}, 100, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 5, 0}, 6, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_MIN_LENGTH, 0, 0, 5, 0, 0}, 6, UINT_MAX, 0, 0, 0},
 
-    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1}, 5, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1, 0}, 5, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2}, 4, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2, 0}, 4, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 2},
+    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 2, 0},
                 10, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2},
+    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2, 0},
                 4, UINT_MAX, 0, 0, 0},
-    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2},
+    {"abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2, 0},
                 4, 6, 0, 0, 0},
 
-    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1}, 5, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1, 0}, 5, UINT_MAX, 0, 0, 0},
-    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2}, 4, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2, 0}, 4, UINT_MAX, 0, 0, 0},
-    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 2},
+    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 2, 0},
                 10, UINT_MAX, 0, 0, 0},
-    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2},
+    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2, 0},
                 4, UINT_MAX, 0, 0, 0},
-    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2},
+    {"^abc.*def", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2, 0},
                 4, 6, 0, 0, 0},
 
-    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1}, 5, 7, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 1, 0}, 5, 7, 0, 0, 0},
-    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2}, 4, 8, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE, 0, 0, 0, 2, 0}, 4, 8, 0, 0, 0},
-    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 8, 2},
+    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 8, 2, 0},
                 8, 8, 0, 0, 0},
-    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2},
+    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 2, 0},
                 4, 8, 0, 0, 0},
-    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2},
+    {"^abcdef", {HS_EXT_FLAG_EDIT_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 2, 0},
                 4, 6, 0, 0, 0},
+
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 1}, 6, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 2}, 6, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 5}, 6, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 0, 2},
+                10, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 0, 2},
+                6, UINT_MAX, 0, 0, 0},
+    {"abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 0, 2},
+                6, 6, 0, 0, 0},
+
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 1}, 6, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 2}, 6, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 5}, 6, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 10, 0, 2},
+                10, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 0, 2},
+                6, UINT_MAX, 0, 0, 0},
+    {"^abc.*def", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 0, 2},
+                6, 6, 0, 0, 0},
+
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 1}, 6, 6, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 2}, 6, 6, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE, 0, 0, 0, 0, 5}, 6, 6, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_LENGTH, 0, 0, 6, 0, 2},
+                6, 6, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MIN_OFFSET, 6, 0, 0, 0, 2},
+                6, 6, 0, 0, 0},
+    {"^abcdef", {HS_EXT_FLAG_HAMMING_DISTANCE | HS_EXT_FLAG_MAX_OFFSET, 0, 6, 0, 0, 2},
+                6, 6, 0, 0, 0},
 };
 
 INSTANTIATE_TEST_CASE_P(ExprInfo, ExprInfop, ValuesIn(ei_test));

unit/internal/nfagraph_find_matches.cpp

@@ -213,7 +213,7 @@ TEST_P(MatchesTest, Check) {
     bool utf8 = (t.flags & HS_FLAG_UTF8) > 0;
 
     set<pair<size_t, size_t>> matches;
-    bool success = findMatches(*g, rm, t.input, matches, 0, t.notEod, utf8);
+    bool success = findMatches(*g, rm, t.input, matches, 0, 0, t.notEod, utf8);
     ASSERT_TRUE(success);
 
     set<pair<size_t, size_t>> expected(begin(t.matches), end(t.matches));

util/ExpressionParser.rl

@@ -49,7 +49,8 @@ enum ParamKey {
     PARAM_MIN_OFFSET,
     PARAM_MAX_OFFSET,
     PARAM_MIN_LENGTH,
-    PARAM_EDIT_DISTANCE
+    PARAM_EDIT_DISTANCE,
+    PARAM_HAMM_DISTANCE
 };
 
 %%{
@@ -97,6 +98,10 @@ enum ParamKey {
                 ext->flags |= HS_EXT_FLAG_EDIT_DISTANCE;
                 ext->edit_distance = num;
                 break;
+            case PARAM_HAMM_DISTANCE:
+                ext->flags |= HS_EXT_FLAG_HAMMING_DISTANCE;
+                ext->hamming_distance = num;
+                break;
             case PARAM_NONE:
             default:
                 // No key specified, syntax invalid.
@@ -158,7 +163,8 @@ bool HS_CDECL readExpression(const std::string &input, std::string &expr,
         param = ('min_offset' @{ key = PARAM_MIN_OFFSET; } |
                  'max_offset' @{ key = PARAM_MAX_OFFSET; } |
                  'min_length' @{ key = PARAM_MIN_LENGTH; } |
-                 'edit_distance' @{ key = PARAM_EDIT_DISTANCE; });
+                 'edit_distance' @{ key = PARAM_EDIT_DISTANCE; } |
+                 'hamming_distance' @{ key = PARAM_HAMM_DISTANCE; });
 
         value = (digit @accumulateNum)+ >{num = 0;};
         param_spec = (' '* param '=' value ' '*) >{ key = PARAM_NONE; }