Initial commit of the regex matcher

Wrap glibc’s regex engine to allow matching and group capture in POSIX
extended regular expressions.

It might be worth rewriting this in terms of the C++11 regex engine;
it’s more featureful and more pleasant to use, although it would require
more casting.  (C can’t represent the std::regex type, so I’d need to
use some void pointers.)

11 files changed, 915 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..7cc3631
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,50 @@
+#                                                                   -*- conf -*-
+
+# Editor backup files
+*~
+\#*
+.\#*
+
+# aclocal
+aclocal.m4
+autom4te.cache/
+
+# libtoolize
+ltmain.sh
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
+
+# autoconf
+configure
+
+# autoheader
+config.h.in
+
+# automake
+Makefile.in
+compile
+config.guess
+config.sub
+depcomp
+install-sh
+missing
+
+# configure
+.deps/
+Makefile
+config.h
+config.log
+config.status
+libtool
+stamp-*
+
+# make
+.dirstamp
+*.la
+*.lo
+*.o
+
+.libs/
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..d645695
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,202 @@
+
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
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+       http://www.apache.org/licenses/LICENSE-2.0
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+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
diff --git a/Makefile.am b/Makefile.am
new file mode 100644
index 0000000..b3926a2
--- /dev/null
+++ b/Makefile.am
@@ -0,0 +1,61 @@
+# Copyright 2015 the Massachusetts Institute of Technology
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License.  You may obtain a copy of
+# the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+ACLOCAL_AMFLAGS = -I m4
+
+CPPFLAGS = \
+	-Wall \
+	-D_FORTIFY_SOURCE=2
+
+CXXFLAGS = \
+	-std=c++11 \
+	-ftrapv \
+	-fstack-protector-strong --param=ssp-buffer-size=4 \
+	-fPIE \
+	-O2 \
+	-ffunction-sections -fdata-sections
+
+CXXFLAGS += \
+	-Weverything \
+	-Wno-c++98-compat \
+	-Wno-padded
+
+LDFLAGS = \
+	-Wl,-Bsymbolic-functions \
+	-fPIE -pie \
+	-Wl,-z,now \
+	-Wl,-z,relro \
+	-Wl,--hash-style=gnu \
+	-Wl,--no-copy-dt-needed-entries \
+	-Wl,--as-needed
+
+lib_LTLIBRARIES = liburweb_regex.la
+
+liburweb_regex_la_SOURCES = src/regex__FFI.cc
+liburweb_regex_la_DATA = \
+	src/lib.urp \
+	src/regex__FFI.h \
+	src/regex__FFI.urs \
+	src/regex.urs \
+	src/regex.ur
+
+liburweb_regex_la_LIBADD = -lurweb
+liburweb_regex_la_LDFLAGS = \
+	-export-symbols-regex '^uw_Regex_'
+
+liburweb_regex_ladir = $(datadir)/urweb/ur/regex
+
+EXTRA_DIST = \
+	LICENSE \
+	$(liburweb_regex_la_DATA)
diff --git a/configure.ac b/configure.ac
new file mode 100644
index 0000000..a9c1c74
--- /dev/null
+++ b/configure.ac
@@ -0,0 +1,48 @@
+# Copyright 2015 the Massachusetts Institute of Technology
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License.  You may obtain a copy of
+# the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+AC_INIT([urweb-regex], [1.0.0], [bbaren@mit.edu])
+AM_INIT_AUTOMAKE([no-dist-gzip dist-xz foreign subdir-objects])
+LT_INIT
+
+AC_CONFIG_MACRO_DIR([m4])
+
+AC_CONFIG_SRCDIR([config.h.in])
+AC_CONFIG_HEADERS([config.h])
+
+AC_PROG_CXX([clang++])
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
+
+AC_CHECK_HEADERS([regex.h],
+  [],
+  [AC_MSG_FAILURE([a POSIX-compatible regex.h is required])])
+
+AC_CHECK_HEADERS([urweb/urweb_cpp.h],
+  [],
+  [AC_MSG_FAILURE([Ur/Web headers are required])])
+save_LIBS=$LIBS
+LIBS="$LIBS -lurweb"
+AC_LINK_IFELSE([AC_LANG_CALL(
+                  [int uw_init_client_data;
+                   int uw_copy_client_data;
+                   int uw_global_custom;
+                   int uw_do_expunge;
+                   int uw_post_expunge;
+                   int uw_free_client_data;],
+                  [uw_error])],
+  [],
+  [AC_MSG_FAILURE([could not link with -lurweb])])
+LIBS=$save_LIBS
+
+AC_OUTPUT([Makefile])
diff --git a/m4/ax_cxx_compile_stdcxx_11.m4 b/m4/ax_cxx_compile_stdcxx_11.m4
new file mode 100644
index 0000000..163a4c6
--- /dev/null
+++ b/m4/ax_cxx_compile_stdcxx_11.m4
@@ -0,0 +1,142 @@
+# ============================================================================
+#  http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html
+# ============================================================================
+#
+# SYNOPSIS
+#
+#   AX_CXX_COMPILE_STDCXX_11([ext|noext],[mandatory|optional])
+#
+# DESCRIPTION
+#
+#   Check for baseline language coverage in the compiler for the C++11
+#   standard; if necessary, add switches to CXXFLAGS to enable support.
+#
+#   The first argument, if specified, indicates whether you insist on an
+#   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
+#   -std=c++11).  If neither is specified, you get whatever works, with
+#   preference for an extended mode.
+#
+#   The second argument, if specified 'mandatory' or if left unspecified,
+#   indicates that baseline C++11 support is required and that the macro
+#   should error out if no mode with that support is found.  If specified
+#   'optional', then configuration proceeds regardless, after defining
+#   HAVE_CXX11 if and only if a supporting mode is found.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
+#   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
+#   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
+#   Copyright (c) 2014 Alexey Sokolov <sokolov@google.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 4
+
+m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [[
+  template <typename T>
+    struct check
+    {
+      static_assert(sizeof(int) <= sizeof(T), "not big enough");
+    };
+
+    struct Base {
+    virtual void f() {}
+    };
+    struct Child : public Base {
+    virtual void f() override {}
+    };
+
+    typedef check<check<bool>> right_angle_brackets;
+
+    int a;
+    decltype(a) b;
+
+    typedef check<int> check_type;
+    check_type c;
+    check_type&& cr = static_cast<check_type&&>(c);
+
+    auto d = a;
+    auto l = [](){};
+]])
+
+AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
+  m4_if([$1], [], [],
+        [$1], [ext], [],
+        [$1], [noext], [],
+        [m4_fatal([invalid argument `$1' to AX_CXX_COMPILE_STDCXX_11])])dnl
+  m4_if([$2], [], [ax_cxx_compile_cxx11_required=true],
+        [$2], [mandatory], [ax_cxx_compile_cxx11_required=true],
+        [$2], [optional], [ax_cxx_compile_cxx11_required=false],
+        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX_11])])
+  AC_LANG_PUSH([C++])dnl
+  ac_success=no
+  AC_CACHE_CHECK(whether $CXX supports C++11 features by default,
+  ax_cv_cxx_compile_cxx11,
+  [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
+    [ax_cv_cxx_compile_cxx11=yes],
+    [ax_cv_cxx_compile_cxx11=no])])
+  if test x$ax_cv_cxx_compile_cxx11 = xyes; then
+    ac_success=yes
+  fi
+
+  m4_if([$1], [noext], [], [dnl
+  if test x$ac_success = xno; then
+    for switch in -std=gnu++11 -std=gnu++0x; do
+      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
+      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
+                     $cachevar,
+        [ac_save_CXXFLAGS="$CXXFLAGS"
+         CXXFLAGS="$CXXFLAGS $switch"
+         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
+          [eval $cachevar=yes],
+          [eval $cachevar=no])
+         CXXFLAGS="$ac_save_CXXFLAGS"])
+      if eval test x\$$cachevar = xyes; then
+        CXXFLAGS="$CXXFLAGS $switch"
+        ac_success=yes
+        break
+      fi
+    done
+  fi])
+
+  m4_if([$1], [ext], [], [dnl
+  if test x$ac_success = xno; then
+    for switch in -std=c++11 -std=c++0x; do
+      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
+      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
+                     $cachevar,
+        [ac_save_CXXFLAGS="$CXXFLAGS"
+         CXXFLAGS="$CXXFLAGS $switch"
+         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
+          [eval $cachevar=yes],
+          [eval $cachevar=no])
+         CXXFLAGS="$ac_save_CXXFLAGS"])
+      if eval test x\$$cachevar = xyes; then
+        CXXFLAGS="$CXXFLAGS $switch"
+        ac_success=yes
+        break
+      fi
+    done
+  fi])
+  AC_LANG_POP([C++])
+  if test x$ax_cxx_compile_cxx11_required = xtrue; then
+    if test x$ac_success = xno; then
+      AC_MSG_ERROR([*** A compiler with support for C++11 language features is required.])
+    fi
+  else
+    if test x$ac_success = xno; then
+      HAVE_CXX11=0
+      AC_MSG_NOTICE([No compiler with C++11 support was found])
+    else
+      HAVE_CXX11=1
+      AC_DEFINE(HAVE_CXX11,1,
+                [define if the compiler supports basic C++11 syntax])
+    fi
+
+    AC_SUBST(HAVE_CXX11)
+  fi
+])
diff --git a/src/lib.urp b/src/lib.urp
new file mode 100644
index 0000000..9f95450
--- /dev/null
+++ b/src/lib.urp
@@ -0,0 +1,5 @@
+ffi regex__FFI
+include regex__FFI.h
+link -lurweb_regex
+
+regex
\ No newline at end of file
diff --git a/src/regex.ur b/src/regex.ur
new file mode 100644
index 0000000..ddc7793
--- /dev/null
+++ b/src/regex.ur
@@ -0,0 +1,44 @@
+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+structure FFI = Regex__FFI
+
+type t = FFI.regex
+
+val compile = FFI.compile True
+
+val compile_case_insensitive = FFI.compile False
+
+fun match regex input =
+  (* Perform the match. *)
+  let
+    val result = FFI.do_match regex input
+  in
+    if not (FFI.succeeded result)
+    then
+      (* No match occurred. *)
+      None
+    else
+      (* Get the subexpressions.  We must do this iteratively, as the Regex__FFI
+      API can't return a list of matches. *)
+      let
+        fun loop i =
+          if i = FFI.n_subexpression_matches result
+          then
+            (* We've got all the subexpressions. *)
+            []
+          else FFI.subexpression_match result i :: loop (i + 1)
+      in
+        Some (loop 0)
+      end
+  end
diff --git a/src/regex.urs b/src/regex.urs
new file mode 100644
index 0000000..15ce216
--- /dev/null
+++ b/src/regex.urs
@@ -0,0 +1,77 @@
+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+(* Regular expression matching
+
+This library implements POSIX extended regular expressions, which most closely
+match what 'normal' people think about when they hear 'regular expressions'.
+Here's a brief syntax reminder:
+
+  .[]^$()\*{}?+| are metacharacters and must be backslash-escaped if you want to
+  use them.  (Remember, in Ur/Web, backslash is also the string escape
+  character, so if you want to match a literal open brace, you need to specify
+  "\\{", if you want to match a literal backslash, you need to specify "\\\\",
+  etc.)
+
+  . matches any character
+  x? matches 'x' zero or one time
+  x* matches 'x' zero or more times
+  x+ matches 'x' one or more times
+  x{3,5} matches 'xxx', 'xxxx', and 'xxxxx'
+
+  ^ matches the start of a line
+  $ matches the end of a line
+
+  [abcx-z] matches 'a', 'b', 'c', 'x', 'y', or 'z'
+  [^a-z] matches any single character not equal to 'a', 'b', ..., or 'z'
+
+  (abc) matches the string 'abc' and saves it as a marked subexpression
+  \3 matches the 3rd marked subexpression
+
+  Character classes may be used inside bracket expressions:
+  [:alnum:]   [A-Za-z0-9]                         alphanumeric characters
+  [:alpha:]   [A-Za-z]                            alphabetic characters
+  [:blank:]   [ \t]                               space and tab
+  [:cntrl:]   [\x00-\x1F\x7F]                     control characters
+  [:digit:]   [0-9]                               digits
+  [:graph:]   [\x21-\x7E]                         visible characters
+  [:lower:]   [a-z]                               lowercase letters
+  [:print:]   [\x20-\x7E]                         visible characters and the space character
+  [:punct:]   [][!"#$%&'()*+,./:;<=>?@\^_`{|}~-]  punctuation characters
+  [:space:]   [ \t\r\n\v\f]                       whitespace characters
+  [:upper:]   [A-Z]                               uppercase letters
+  [:xdigit:]  [A-Fa-f0-9]                         Hexadecimal digits
+  So if you want to match all duodecimal digits, you can specify
+  '[[:digit:]A-Ba-b]'.  If you simply want all decimal digits, you need
+  '[[:digit:]]'. *)
+
+
+(* Creating *)
+
+(* A compiled regular expression. *)
+type t
+
+(* Compiles a regular expression from a POSIX extended regular expression
+string. *)
+val compile : string -> t
+
+(* Compiles a case-insensitive regular expression from a POSIX extended regular expression string. *)
+val compile_case_insensitive : string -> t
+
+
+(* Searching *)
+
+(* Matches a regular expression against any part of a string.  Returns 'Some
+strs', where 'strs' is a list of subexpression matches, if a match succeeds, and
+'None' otherwise. *)
+val match : t -> string -> option (list string)
diff --git a/src/regex__FFI.cc b/src/regex__FFI.cc
new file mode 100644
index 0000000..403171f
--- /dev/null
+++ b/src/regex__FFI.cc
@@ -0,0 +1,192 @@
+// Copyright (C) 2015 the Massachusetts Institute of Technology
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License.  You may obtain a copy
+// of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+#include "regex__FFI.h"
+
+#include <sys/types.h>
+#include <regex.h>
+
+#include <cstring>
+
+extern "C" {
+#include <urweb/urweb_cpp.h>
+}
+
+#include "config.h"
+
+namespace {
+
+using Regex = uw_Regex__FFI_regex;
+using Match = uw_Regex__FFI_match;
+
+// Asserts a condition without crashing or releasing information about where the
+// error occurred.  This function is essential for web programming, where an
+// attacker should not be able to bring down the app by causing an assertion
+// failure.
+void Assert(uw_context* const context, const bool condition,
+            const failure_kind action, const char* const message) {
+  if (!condition) {
+    uw_error(context, action, message);
+  }
+}
+
+void Assert(uw_context* const context,
+            const bool condition, const char* const message) {
+  Assert(context, condition, FATAL, message);
+}
+
+void FinalizeRegex(void* regex, [[gnu::unused]] const int _will_retry) {
+  regfree(reinterpret_cast<regex_t*>(regex));
+}
+
+void DeleteRegex(void* regex, [[gnu::unused]] const int _will_retry) {
+  delete reinterpret_cast<regex_t*>(regex);
+}
+
+}  // namespace
+
+uw_Basis_bool uw_Regex__FFI_succeeded(
+    [[gnu::unused]] struct uw_context* _context,
+    const Match match) {
+  return match.succeeded ? uw_Basis_True : uw_Basis_False;
+}
+
+uw_Basis_int uw_Regex__FFI_n_subexpression_matches(
+    [[gnu::unused]] struct uw_context* _context,
+    const Match match) {
+  return match.n_matches;
+}
+
+uw_Basis_string uw_Regex__FFI_subexpression_match(
+    struct uw_context* context,
+    const Match match,
+    const uw_Basis_int match_index) {
+  Assert(context, match.matches[match_index].rm_so != -1,
+         "regex: match does not exist");
+  // Locate the substring in the string to match aginst.
+  const char* const substring_start =
+      match.haystack + match.matches[match_index].rm_so;
+  // Copy it into its own buffer so we can properly null-terminate it.
+  const std::size_t substring_length =
+      static_cast<std::size_t>(match.matches[match_index].rm_eo
+                               - match.matches[match_index].rm_so);
+  uw_Basis_string result = reinterpret_cast<uw_Basis_string>(
+      uw_malloc(context, substring_length + 1));
+  std::memcpy(result, substring_start, substring_length);
+  result[substring_length] = '\0';
+  return result;
+}
+
+Regex uw_Regex__FFI_compile(uw_context* const context,
+                            const uw_Basis_bool case_sensitive,
+                            const uw_Basis_string input) {
+  Regex result;
+  result.text = input;
+  // We'd like to stack-allocate the compiled field of the Regex struct--or, at
+  // least, to allocate it with uw_malloc.  Unfortunately, neither of those will
+  // work, because we need to be able to run a finalizer on it, and Ur
+  // finalizers can only reference addresses that are not managed by Ur.
+  result.compiled = new regex_t;
+  Assert(context,
+         uw_register_transactional(context, result.compiled,
+                                   nullptr, nullptr, DeleteRegex) == 0,
+         "regex: could not register DeleteRegex finalizer");
+  // Compile the regex.
+  const auto flags = REG_EXTENDED | (case_sensitive ? 0 : REG_ICASE);
+  switch (const auto regcomp_error = regcomp(result.compiled, input, flags)) {
+    case 0:
+      // Everything worked perfectly.
+      break;
+    case REG_ESPACE:
+      // We ran out of memory.
+      uw_error(context, BOUNDED_RETRY, "regex: could not allocate");
+    default:
+      // Something else happened.  Generate a nice message for the user.
+      const auto message_size =
+          regerror(regcomp_error, result.compiled, nullptr, 0);
+      char* const message =
+          reinterpret_cast<char*>(uw_malloc(context, message_size));
+      Assert(context,
+             regerror(regcomp_error, result.compiled, message,
+                      message_size) == message_size,
+             "regex: compilation failed, but error message could not be"
+             " generated");
+      uw_error(context, FATAL, "regex: compilation failed: %s", message);
+  }
+  Assert(context,
+         uw_register_transactional(context, result.compiled,
+                                   nullptr, nullptr, FinalizeRegex) == 0,
+         "regex: could not register FinalizeRegex finalizer");
+  // Give the caller the regex.
+  return result;
+}
+
+Match uw_Regex__FFI_do_match(uw_context* const context, const Regex needle,
+                             const uw_Basis_string haystack) {
+  Match result;
+  // Make a duplicate of the string to match against, so if it goes out of scope
+  // in the calling Ur code, we still have it.  TODO(bbaren): Is this necessary?
+  result.haystack =
+      reinterpret_cast<uw_Basis_string>(
+          uw_malloc(context, std::strlen(haystack)));
+  std::strcpy(result.haystack, haystack);
+  // Figure out how many groups we could have so we can allocate enough space to
+  // store the match information.
+  result.n_matches = 0;
+  for (std::size_t i = 0; i < std::strlen(needle.text); i++) {
+    switch (needle.text[i]) {
+      case '\\':
+        // The next character is escaped, so it can't possibly be the
+        // metacharacter '('.  Skip it.
+        i++;
+        break;
+      case '(':
+        // That's our metacharacter.
+        result.n_matches++;
+        break;
+      default:
+        // Nothing interesting.
+        break;
+    }
+  }
+  // Allocate to store the match information.  Allocate one more slot than we
+  // need, because the regex engine puts information about the entire match in
+  // the first slot.
+  result.matches =
+      reinterpret_cast<regmatch_t*>(
+          uw_malloc(context, (result.n_matches + 1) * sizeof(regmatch_t)));
+  // Execute the regex.
+  switch (regexec(needle.compiled, haystack, result.n_matches + 1,
+                  result.matches, 0)) {
+    case 0:
+      // A match occurred.
+      result.succeeded = 1;
+      // Bump the matches array to skip information about the entire match.
+      result.matches++;
+      break;
+    case REG_NOMATCH:
+      // No match occurred.
+      result.succeeded = 0;
+      result.n_matches = 0;
+      result.matches = nullptr;
+      break;
+    case REG_ESPACE:
+      // We ran out of memory.
+      uw_error(context, BOUNDED_RETRY, "regex: could not allocate");
+    default:
+      // Some unknown error occurred.
+      uw_error(context, FATAL, "regex: could not execute regular expression");
+  }
+  return result;
+}
diff --git a/src/regex__FFI.h b/src/regex__FFI.h
new file mode 100644
index 0000000..ff2f13d
--- /dev/null
+++ b/src/regex__FFI.h
@@ -0,0 +1,60 @@
+/* Copyright (C) 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. */
+
+#ifndef URWEB_REGEX__FFI_H
+#define URWEB_REGEX__FFI_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stddef.h>
+
+#include <regex.h>
+#include <urweb/urweb_cpp.h>
+
+typedef struct {
+  char* text;
+  regex_t* compiled;
+} uw_Regex__FFI_regex;
+
+typedef struct {
+  char* haystack;
+  int succeeded;
+  unsigned n_matches;
+  regmatch_t* matches;
+} uw_Regex__FFI_match;
+
+uw_Basis_bool uw_Regex__FFI_succeeded(struct uw_context*,
+                                      const uw_Regex__FFI_match);
+
+uw_Basis_int uw_Regex__FFI_n_subexpression_matches(struct uw_context*,
+                                                   const uw_Regex__FFI_match);
+
+uw_Basis_string uw_Regex__FFI_subexpression_match(struct uw_context*,
+                                                  const uw_Regex__FFI_match,
+                                                  const uw_Basis_int);
+
+uw_Regex__FFI_regex uw_Regex__FFI_compile(struct uw_context*,
+                                          const uw_Basis_bool,
+                                          const uw_Basis_string);
+
+uw_Regex__FFI_match uw_Regex__FFI_do_match(struct uw_context*,
+                                           const uw_Regex__FFI_regex,
+                                           const uw_Basis_string);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // URWEB_REGEX__FFI_H
diff --git a/src/regex__FFI.urs b/src/regex__FFI.urs
new file mode 100644
index 0000000..690ca1d
--- /dev/null
+++ b/src/regex__FFI.urs
@@ -0,0 +1,34 @@
+(* Copyright 2015 the Massachusetts Institute of Technology
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License.  You may obtain a copy of the
+License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+(* This is an internal module.  You should use the high-level API in Regex
+instead. *)
+
+
+(* A compiled regular expression. *)
+type regex
+
+(* Data about a match.  There is no function which returns all subexpression
+matches, as we can't build an Ur list in C. *)
+type match
+val succeeded : match -> bool
+val n_subexpression_matches : match -> int
+val subexpression_match : match -> int -> string
+
+
+(* Compiles a regular expression from a POSIX extended regular expression
+string. *)
+val compile : bool (* case sensitive? *) -> string -> regex
+
+(* Matches a regular expression against any part of a string. *)
+val do_match : regex -> string -> match