diff --git a/externals/porter-stemmer/LICENSE b/externals/porter-stemmer/LICENSE
new file mode 100644
index 0000000000..d4afc6cfa7
--- /dev/null
+++ b/externals/porter-stemmer/LICENSE
@@ -0,0 +1,20 @@
+The MIT License (MIT)
+
+Copyright (c) 2005-2016 Richard Heyes (http://www.phpguru.org/)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/externals/porter-stemmer/README.md b/externals/porter-stemmer/README.md
new file mode 100644
index 0000000000..06bc417f9f
--- /dev/null
+++ b/externals/porter-stemmer/README.md
@@ -0,0 +1,42 @@
+# Porter Stemmer by Richard Heyes
+
+# Installation (with composer)
+
+```json
+{
+	"require": {
+		"camspiers/porter-stemmer": "1.0.0"
+	}
+}
+```
+
+	$ composer install
+
+# Usage
+
+```php
+$stem = Porter::Stem($word);
+```
+
+# License
+
+The MIT License (MIT)
+
+Copyright (c) 2005-2016 Richard Heyes (http://www.phpguru.org/)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/externals/porter-stemmer/src/Porter.php b/externals/porter-stemmer/src/Porter.php
new file mode 100644
index 0000000000..0720c53ba0
--- /dev/null
+++ b/externals/porter-stemmer/src/Porter.php
@@ -0,0 +1,426 @@
+<?php
+
+# vim: set expandtab tabstop=4 shiftwidth=4 softtabstop=4:
+
+/**
+ * Copyright (c) 2005-2016 Richard Heyes (http://www.phpguru.org/)
+ *
+ * Portions Copyright 2003-2007 Jon Abernathy <jon@chuggnutt.com>
+ *
+ * Originally available under the GPL 2 or greater. Relicensed with permission
+ * of original authors under the MIT License in 2016.
+ *
+ * All rights reserved.
+ *
+ * @package   PorterStemmer
+ * @author    Richard Heyes
+ * @author    Jon Abernathy <jon@chuggnutt.com>
+ * @copyright 2005-2016 Richard Heyes (http://www.phpguru.org/)
+ * @license   http://www.opensource.org/licenses/mit-license.html MIT License
+ */
+
+/**
+ * PHP 5 Implementation of the Porter Stemmer algorithm. Certain elements
+ * were borrowed from the (broken) implementation by Jon Abernathy.
+ *
+ * See http://tartarus.org/~martin/PorterStemmer/ for a description of the
+ * algorithm.
+ *
+ * Usage:
+ *
+ *  $stem = PorterStemmer::Stem($word);
+ *
+ * How easy is that?
+ *
+ * @package   PorterStemmer
+ * @author    Richard Heyes
+ * @author    Jon Abernathy <jon@chuggnutt.com>
+ * @copyright 2005-2016 Richard Heyes (http://www.phpguru.org/)
+ * @license   http://www.opensource.org/licenses/mit-license.html MIT License
+ */
+class Porter
+{
+    /**
+     * Regex for matching a consonant
+     *
+     * @var string
+     */
+    private static $regex_consonant = '(?:[bcdfghjklmnpqrstvwxz]|(?<=[aeiou])y|^y)';
+
+    /**
+     * Regex for matching a vowel
+     *
+     * @var string
+     */
+    private static $regex_vowel = '(?:[aeiou]|(?<![aeiou])y)';
+
+    /**
+     * Stems a word. Simple huh?
+     *
+     * @param string $word Word to stem
+     *
+     * @return string Stemmed word
+     */
+    public static function Stem($word)
+    {
+        if (strlen($word) <= 2) {
+            return $word;
+        }
+
+        $word = self::step1ab($word);
+        $word = self::step1c($word);
+        $word = self::step2($word);
+        $word = self::step3($word);
+        $word = self::step4($word);
+        $word = self::step5($word);
+
+        return $word;
+    }
+
+    /**
+     * Step 1
+     */
+    private static function step1ab($word)
+    {
+        // Part a
+        if (substr($word, -1) == 's') {
+
+               self::replace($word, 'sses', 'ss')
+            OR self::replace($word, 'ies', 'i')
+            OR self::replace($word, 'ss', 'ss')
+            OR self::replace($word, 's', '');
+        }
+
+        // Part b
+        if (substr($word, -2, 1) != 'e' OR !self::replace($word, 'eed', 'ee', 0)) { // First rule
+            $v = self::$regex_vowel;
+
+            // ing and ed
+            if (   preg_match("#$v+#", substr($word, 0, -3)) && self::replace($word, 'ing', '')
+                OR preg_match("#$v+#", substr($word, 0, -2)) && self::replace($word, 'ed', '')) { // Note use of && and OR, for precedence reasons
+
+                // If one of above two test successful
+                if (    !self::replace($word, 'at', 'ate')
+                    AND !self::replace($word, 'bl', 'ble')
+                    AND !self::replace($word, 'iz', 'ize')) {
+
+                    // Double consonant ending
+                    if (    self::doubleConsonant($word)
+                        AND substr($word, -2) != 'll'
+                        AND substr($word, -2) != 'ss'
+                        AND substr($word, -2) != 'zz') {
+
+                        $word = substr($word, 0, -1);
+
+                    } elseif (self::m($word) == 1 AND self::cvc($word)) {
+                        $word .= 'e';
+                    }
+                }
+            }
+        }
+
+        return $word;
+    }
+
+    /**
+     * Step 1c
+     *
+     * @param string $word Word to stem
+     */
+    private static function step1c($word)
+    {
+        $v = self::$regex_vowel;
+
+        if (substr($word, -1) == 'y' && preg_match("#$v+#", substr($word, 0, -1))) {
+            self::replace($word, 'y', 'i');
+        }
+
+        return $word;
+    }
+
+    /**
+     * Step 2
+     *
+     * @param string $word Word to stem
+     */
+    private static function step2($word)
+    {
+        switch (substr($word, -2, 1)) {
+            case 'a':
+                   self::replace($word, 'ational', 'ate', 0)
+                OR self::replace($word, 'tional', 'tion', 0);
+                break;
+
+            case 'c':
+                   self::replace($word, 'enci', 'ence', 0)
+                OR self::replace($word, 'anci', 'ance', 0);
+                break;
+
+            case 'e':
+                self::replace($word, 'izer', 'ize', 0);
+                break;
+
+            case 'g':
+                self::replace($word, 'logi', 'log', 0);
+                break;
+
+            case 'l':
+                   self::replace($word, 'entli', 'ent', 0)
+                OR self::replace($word, 'ousli', 'ous', 0)
+                OR self::replace($word, 'alli', 'al', 0)
+                OR self::replace($word, 'bli', 'ble', 0)
+                OR self::replace($word, 'eli', 'e', 0);
+                break;
+
+            case 'o':
+                   self::replace($word, 'ization', 'ize', 0)
+                OR self::replace($word, 'ation', 'ate', 0)
+                OR self::replace($word, 'ator', 'ate', 0);
+                break;
+
+            case 's':
+                   self::replace($word, 'iveness', 'ive', 0)
+                OR self::replace($word, 'fulness', 'ful', 0)
+                OR self::replace($word, 'ousness', 'ous', 0)
+                OR self::replace($word, 'alism', 'al', 0);
+                break;
+
+            case 't':
+                   self::replace($word, 'biliti', 'ble', 0)
+                OR self::replace($word, 'aliti', 'al', 0)
+                OR self::replace($word, 'iviti', 'ive', 0);
+                break;
+        }
+
+        return $word;
+    }
+
+    /**
+     * Step 3
+     *
+     * @param string $word String to stem
+     */
+    private static function step3($word)
+    {
+        switch (substr($word, -2, 1)) {
+            case 'a':
+                self::replace($word, 'ical', 'ic', 0);
+                break;
+
+            case 's':
+                self::replace($word, 'ness', '', 0);
+                break;
+
+            case 't':
+                   self::replace($word, 'icate', 'ic', 0)
+                OR self::replace($word, 'iciti', 'ic', 0);
+                break;
+
+            case 'u':
+                self::replace($word, 'ful', '', 0);
+                break;
+
+            case 'v':
+                self::replace($word, 'ative', '', 0);
+                break;
+
+            case 'z':
+                self::replace($word, 'alize', 'al', 0);
+                break;
+        }
+
+        return $word;
+    }
+
+    /**
+     * Step 4
+     *
+     * @param string $word Word to stem
+     */
+    private static function step4($word)
+    {
+        switch (substr($word, -2, 1)) {
+            case 'a':
+                self::replace($word, 'al', '', 1);
+                break;
+
+            case 'c':
+                   self::replace($word, 'ance', '', 1)
+                OR self::replace($word, 'ence', '', 1);
+                break;
+
+            case 'e':
+                self::replace($word, 'er', '', 1);
+                break;
+
+            case 'i':
+                self::replace($word, 'ic', '', 1);
+                break;
+
+            case 'l':
+                   self::replace($word, 'able', '', 1)
+                OR self::replace($word, 'ible', '', 1);
+                break;
+
+            case 'n':
+                   self::replace($word, 'ant', '', 1)
+                OR self::replace($word, 'ement', '', 1)
+                OR self::replace($word, 'ment', '', 1)
+                OR self::replace($word, 'ent', '', 1);
+                break;
+
+            case 'o':
+                if (substr($word, -4) == 'tion' OR substr($word, -4) == 'sion') {
+                   self::replace($word, 'ion', '', 1);
+                } else {
+                    self::replace($word, 'ou', '', 1);
+                }
+                break;
+
+            case 's':
+                self::replace($word, 'ism', '', 1);
+                break;
+
+            case 't':
+                   self::replace($word, 'ate', '', 1)
+                OR self::replace($word, 'iti', '', 1);
+                break;
+
+            case 'u':
+                self::replace($word, 'ous', '', 1);
+                break;
+
+            case 'v':
+                self::replace($word, 'ive', '', 1);
+                break;
+
+            case 'z':
+                self::replace($word, 'ize', '', 1);
+                break;
+        }
+
+        return $word;
+    }
+
+    /**
+     * Step 5
+     *
+     * @param string $word Word to stem
+     */
+    private static function step5($word)
+    {
+        // Part a
+        if (substr($word, -1) == 'e') {
+            if (self::m(substr($word, 0, -1)) > 1) {
+                self::replace($word, 'e', '');
+
+            } elseif (self::m(substr($word, 0, -1)) == 1) {
+
+                if (!self::cvc(substr($word, 0, -1))) {
+                    self::replace($word, 'e', '');
+                }
+            }
+        }
+
+        // Part b
+        if (self::m($word) > 1 AND self::doubleConsonant($word) AND substr($word, -1) == 'l') {
+            $word = substr($word, 0, -1);
+        }
+
+        return $word;
+    }
+
+    /**
+     * Replaces the first string with the second, at the end of the string
+     *
+     * If third arg is given, then the preceding string must match that m
+     * count at least.
+     *
+     * @param  string $str   String to check
+     * @param  string $check Ending to check for
+     * @param  string $repl  Replacement string
+     * @param  int    $m     Optional minimum number of m() to meet
+     *
+     * @return bool Whether the $check string was at the end of the $str
+     *              string. True does not necessarily mean that it was
+     *              replaced.
+     */
+    private static function replace(&$str, $check, $repl, $m = null)
+    {
+        $len = 0 - strlen($check);
+
+        if (substr($str, $len) == $check) {
+            $substr = substr($str, 0, $len);
+            if (is_null($m) OR self::m($substr) > $m) {
+                $str = $substr . $repl;
+            }
+
+            return true;
+        }
+
+        return false;
+    }
+
+    /**
+     * What, you mean it's not obvious from the name?
+     *
+     * m() measures the number of consonant sequences in $str. if c is
+     * a consonant sequence and v a vowel sequence, and <..> indicates arbitrary
+     * presence,
+     *
+     * <c><v>       gives 0
+     * <c>vc<v>     gives 1
+     * <c>vcvc<v>   gives 2
+     * <c>vcvcvc<v> gives 3
+     *
+     * @param  string $str The string to return the m count for
+     *
+     * @return int The m count
+     */
+    private static function m($str)
+    {
+        $c = self::$regex_consonant;
+        $v = self::$regex_vowel;
+
+        $str = preg_replace("#^$c+#", '', $str);
+        $str = preg_replace("#$v+$#", '', $str);
+
+        preg_match_all("#($v+$c+)#", $str, $matches);
+
+        return count($matches[1]);
+    }
+
+    /**
+     * Returns true/false as to whether the given string contains two
+     * of the same consonant next to each other at the end of the string.
+     *
+     * @param  string $str String to check
+     *
+     * @return bool Result
+     */
+    private static function doubleConsonant($str)
+    {
+        $c = self::$regex_consonant;
+
+        return preg_match("#$c{2}$#", $str, $matches) AND $matches[0]{0} == $matches[0]{1};
+    }
+
+    /**
+     * Checks for ending CVC sequence where second C is not W, X or Y
+     *
+     * @param  string $str String to check
+     *
+     * @return bool Result
+     */
+    private static function cvc($str)
+    {
+        $c = self::$regex_consonant;
+        $v = self::$regex_vowel;
+
+        return     preg_match("#($c$v$c)$#", $str, $matches)
+               AND strlen($matches[1]) == 3
+               AND $matches[1]{2} != 'w'
+               AND $matches[1]{2} != 'x'
+               AND $matches[1]{2} != 'y';
+    }
+}
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 174c0e4876..9b15a81c49 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -5658,6 +5658,12 @@ phutil_register_library_map(array(
     'PhutilRemarkupTableBlockRule' => 'infrastructure/markup/blockrule/PhutilRemarkupTableBlockRule.php',
     'PhutilRemarkupTestInterpreterRule' => 'infrastructure/markup/blockrule/PhutilRemarkupTestInterpreterRule.php',
     'PhutilRemarkupUnderlineRule' => 'infrastructure/markup/markuprule/PhutilRemarkupUnderlineRule.php',
+    'PhutilSearchQueryCompiler' => 'applications/search/compiler/PhutilSearchQueryCompiler.php',
+    'PhutilSearchQueryCompilerSyntaxException' => 'applications/search/compiler/PhutilSearchQueryCompilerSyntaxException.php',
+    'PhutilSearchQueryCompilerTestCase' => 'applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php',
+    'PhutilSearchQueryToken' => 'applications/search/compiler/PhutilSearchQueryToken.php',
+    'PhutilSearchStemmer' => 'applications/search/compiler/PhutilSearchStemmer.php',
+    'PhutilSearchStemmerTestCase' => 'applications/search/compiler/__tests__/PhutilSearchStemmerTestCase.php',
     'PhutilSlackAuthAdapter' => 'applications/auth/adapter/PhutilSlackAuthAdapter.php',
     'PhutilTwitchAuthAdapter' => 'applications/auth/adapter/PhutilTwitchAuthAdapter.php',
     'PhutilTwitterAuthAdapter' => 'applications/auth/adapter/PhutilTwitterAuthAdapter.php',
@@ -12483,6 +12489,12 @@ phutil_register_library_map(array(
     'PhutilRemarkupTableBlockRule' => 'PhutilRemarkupBlockRule',
     'PhutilRemarkupTestInterpreterRule' => 'PhutilRemarkupBlockInterpreter',
     'PhutilRemarkupUnderlineRule' => 'PhutilRemarkupRule',
+    'PhutilSearchQueryCompiler' => 'Phobject',
+    'PhutilSearchQueryCompilerSyntaxException' => 'Exception',
+    'PhutilSearchQueryCompilerTestCase' => 'PhutilTestCase',
+    'PhutilSearchQueryToken' => 'Phobject',
+    'PhutilSearchStemmer' => 'Phobject',
+    'PhutilSearchStemmerTestCase' => 'PhutilTestCase',
     'PhutilSlackAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilTwitchAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilTwitterAuthAdapter' => 'PhutilOAuth1AuthAdapter',
diff --git a/src/applications/search/compiler/PhutilSearchQueryCompiler.php b/src/applications/search/compiler/PhutilSearchQueryCompiler.php
new file mode 100644
index 0000000000..eb5ee860c9
--- /dev/null
+++ b/src/applications/search/compiler/PhutilSearchQueryCompiler.php
@@ -0,0 +1,374 @@
+<?php
+
+final class PhutilSearchQueryCompiler
+  extends Phobject {
+
+  private $operators = '+ -><()~*:""&|';
+  private $query;
+  private $stemmer;
+  private $enableFunctions = false;
+
+  const OPERATOR_NOT = 'not';
+  const OPERATOR_AND = 'and';
+  const OPERATOR_SUBSTRING = 'sub';
+  const OPERATOR_EXACT = 'exact';
+
+  public function setOperators($operators) {
+    $this->operators = $operators;
+    return $this;
+  }
+
+  public function getOperators() {
+    return $this->operators;
+  }
+
+  public function setStemmer(PhutilSearchStemmer $stemmer) {
+    $this->stemmer = $stemmer;
+    return $this;
+  }
+
+  public function getStemmer() {
+    return $this->stemmer;
+  }
+
+  public function setEnableFunctions($enable_functions) {
+    $this->enableFunctions = $enable_functions;
+    return $this;
+  }
+
+  public function getEnableFunctions() {
+    return $this->enableFunctions;
+  }
+
+  public function compileQuery(array $tokens) {
+    assert_instances_of($tokens, 'PhutilSearchQueryToken');
+
+    $result = array();
+    foreach ($tokens as $token) {
+      $result[] = $this->renderToken($token);
+    }
+
+    return $this->compileRenderedTokens($result);
+  }
+
+  public function compileLiteralQuery(array $tokens) {
+    assert_instances_of($tokens, 'PhutilSearchQueryToken');
+
+    $result = array();
+    foreach ($tokens as $token) {
+      if (!$token->isQuoted()) {
+        continue;
+      }
+      $result[] = $this->renderToken($token);
+    }
+
+    return $this->compileRenderedTokens($result);
+  }
+
+  public function compileStemmedQuery(array $tokens) {
+    assert_instances_of($tokens, 'PhutilSearchQueryToken');
+
+    $result = array();
+    foreach ($tokens as $token) {
+      if ($token->isQuoted()) {
+        continue;
+      }
+      $result[] = $this->renderToken($token, $this->getStemmer());
+    }
+
+    return $this->compileRenderedTokens($result);
+  }
+
+  private function compileRenderedTokens(array $list) {
+    if (!$list) {
+      return null;
+    }
+
+    $list = array_unique($list);
+    return implode(' ', $list);
+  }
+
+  public function newTokens($query) {
+    $results = $this->tokenizeQuery($query);
+
+    $tokens = array();
+    foreach ($results as $result) {
+      $tokens[] = PhutilSearchQueryToken::newFromDictionary($result);
+    }
+
+    return $tokens;
+  }
+
+  private function tokenizeQuery($query) {
+    $maximum_bytes = 1024;
+
+    $query_bytes = strlen($query);
+    if ($query_bytes > $maximum_bytes) {
+      throw new PhutilSearchQueryCompilerSyntaxException(
+        pht(
+          'Query is too long (%s bytes, maximum is %s bytes).',
+          new PhutilNumber($query_bytes),
+          new PhutilNumber($maximum_bytes)));
+    }
+
+    $query = phutil_utf8v($query);
+    $length = count($query);
+
+    $enable_functions = $this->getEnableFunctions();
+
+    $mode = 'scan';
+    $current_operator = array();
+    $current_token = array();
+    $current_function = null;
+    $is_quoted = false;
+    $tokens = array();
+
+    if ($enable_functions) {
+      $operator_characters = '[~=+-]';
+    } else {
+      $operator_characters = '[+-]';
+    }
+
+    for ($ii = 0; $ii < $length; $ii++) {
+      $character = $query[$ii];
+
+      if ($mode == 'scan') {
+        if (preg_match('/^\s\z/u', $character)) {
+          continue;
+        }
+
+        $mode = 'function';
+      }
+
+      if ($mode == 'function') {
+        $mode = 'operator';
+
+        if ($enable_functions) {
+          $found = false;
+          for ($jj = $ii; $jj < $length; $jj++) {
+            if (preg_match('/^[a-zA-Z]\z/u', $query[$jj])) {
+              continue;
+            }
+            if ($query[$jj] == ':') {
+              $found = $jj;
+            }
+            break;
+          }
+
+          if ($found !== false) {
+            $function = array_slice($query, $ii, ($jj - $ii));
+            $current_function = implode('', $function);
+
+            if (!strlen($current_function)) {
+              $current_function = null;
+            }
+
+            $ii = $jj;
+            continue;
+          }
+        }
+      }
+
+      if ($mode == 'operator') {
+        if (preg_match('/^\s\z/u', $character)) {
+          continue;
+        }
+
+        if (preg_match('/^'.$operator_characters.'\z/', $character)) {
+          $current_operator[] = $character;
+          continue;
+        }
+
+        $mode = 'quote';
+      }
+
+      if ($mode == 'quote') {
+        if (preg_match('/^"\z/', $character)) {
+          $is_quoted = true;
+          $mode = 'token';
+          continue;
+        }
+
+        $mode = 'token';
+      }
+
+      if ($mode == 'token') {
+        $capture = false;
+        $was_quoted = $is_quoted;
+        if ($is_quoted) {
+          if (preg_match('/^"\z/', $character)) {
+            $capture = true;
+            $mode = 'scan';
+            $is_quoted = false;
+          }
+        } else {
+          if (preg_match('/^\s\z/u', $character)) {
+            $capture = true;
+            $mode = 'scan';
+          }
+
+          if (preg_match('/^"\z/', $character)) {
+            $capture = true;
+            $mode = 'token';
+            $is_quoted = true;
+          }
+        }
+
+        if ($capture) {
+          $token = array(
+            'operator' => $current_operator,
+            'quoted' => $was_quoted,
+            'value' => $current_token,
+          );
+
+          if ($enable_functions) {
+            $token['function'] = $current_function;
+          }
+
+          $tokens[] = $token;
+
+          $current_operator = array();
+          $current_token = array();
+          $current_function = null;
+          continue;
+        } else {
+          $current_token[] = $character;
+        }
+      }
+    }
+
+    if ($is_quoted) {
+      throw new PhutilSearchQueryCompilerSyntaxException(
+        pht(
+          'Query contains unmatched double quotes.'));
+    }
+
+    if ($mode == 'operator') {
+      throw new PhutilSearchQueryCompilerSyntaxException(
+        pht(
+          'Query contains operator ("%s") with no search term.',
+          implode('', $current_operator)));
+    }
+
+    $token = array(
+      'operator' => $current_operator,
+      'quoted' => false,
+      'value' => $current_token,
+    );
+
+    if ($enable_functions) {
+      $token['function'] = $current_function;
+    }
+
+    $tokens[] = $token;
+
+    $results = array();
+    foreach ($tokens as $token) {
+      $value = implode('', $token['value']);
+      $operator_string = implode('', $token['operator']);
+
+      if (!strlen($value)) {
+        continue;
+      }
+
+      $is_quoted = $token['quoted'];
+
+      switch ($operator_string) {
+        case '-':
+          $operator = self::OPERATOR_NOT;
+          break;
+        case '~':
+          $operator = self::OPERATOR_SUBSTRING;
+          break;
+        case '=':
+          $operator = self::OPERATOR_EXACT;
+          break;
+        case '+':
+          $operator = self::OPERATOR_AND;
+          break;
+        case '':
+          // See T12995. If this query term contains Chinese, Japanese or
+          // Korean characters, treat the term as a substring term by default.
+          // These languages do not separate words with spaces, so the term
+          // search mode is normally useless.
+          if ($enable_functions && !$is_quoted && phutil_utf8_is_cjk($value)) {
+            $operator = self::OPERATOR_SUBSTRING;
+          } else {
+            $operator = self::OPERATOR_AND;
+          }
+          break;
+        default:
+          throw new PhutilSearchQueryCompilerSyntaxException(
+            pht(
+              'Query has an invalid sequence of operators ("%s").',
+              $operator_string));
+      }
+
+      $result = array(
+        'operator' => $operator,
+        'quoted' => $is_quoted,
+        'value' => $value,
+      );
+
+      if ($enable_functions) {
+        $result['function'] = $token['function'];
+      }
+
+      $results[] = $result;
+    }
+
+    return $results;
+  }
+
+  private function renderToken(
+    PhutilSearchQueryToken $token,
+    PhutilSearchStemmer $stemmer = null) {
+    $value = $token->getValue();
+
+    if ($stemmer) {
+      $value = $stemmer->stemToken($value);
+    }
+
+    $value = $this->quoteToken($value);
+    $operator = $token->getOperator();
+    $prefix = $this->getOperatorPrefix($operator);
+
+    $value = $prefix.$value;
+
+    return $value;
+  }
+
+  private function getOperatorPrefix($operator) {
+    $operators = $this->operators;
+
+    switch ($operator) {
+      case self::OPERATOR_AND:
+        $prefix = $operators[0];
+        break;
+      case self::OPERATOR_NOT:
+        $prefix = $operators[2];
+        break;
+      default:
+        throw new PhutilSearchQueryCompilerSyntaxException(
+          pht(
+            'Unsupported operator prefix "%s".',
+            $operator));
+    }
+
+    if ($prefix == ' ') {
+      $prefix = null;
+    }
+
+    return $prefix;
+  }
+
+  private function quoteToken($value) {
+    $operators = $this->operators;
+
+    $open_quote = $this->operators[10];
+    $close_quote = $this->operators[11];
+
+    return $open_quote.$value.$close_quote;
+  }
+
+}
diff --git a/src/applications/search/compiler/PhutilSearchQueryCompilerSyntaxException.php b/src/applications/search/compiler/PhutilSearchQueryCompilerSyntaxException.php
new file mode 100644
index 0000000000..b09d36d04f
--- /dev/null
+++ b/src/applications/search/compiler/PhutilSearchQueryCompilerSyntaxException.php
@@ -0,0 +1,4 @@
+<?php
+
+final class PhutilSearchQueryCompilerSyntaxException
+  extends Exception {}
diff --git a/src/applications/search/compiler/PhutilSearchQueryToken.php b/src/applications/search/compiler/PhutilSearchQueryToken.php
new file mode 100644
index 0000000000..b63427131e
--- /dev/null
+++ b/src/applications/search/compiler/PhutilSearchQueryToken.php
@@ -0,0 +1,37 @@
+<?php
+
+final class PhutilSearchQueryToken extends Phobject {
+
+  private $isQuoted;
+  private $value;
+  private $operator;
+  private $function;
+
+  public static function newFromDictionary(array $dictionary) {
+    $token = new self();
+
+    $token->isQuoted = $dictionary['quoted'];
+    $token->operator = $dictionary['operator'];
+    $token->value = $dictionary['value'];
+    $token->function = idx($dictionary, 'function');
+
+    return $token;
+  }
+
+  public function isQuoted() {
+    return $this->isQuoted;
+  }
+
+  public function getValue() {
+    return $this->value;
+  }
+
+  public function getOperator() {
+    return $this->operator;
+  }
+
+  public function getFunction() {
+    return $this->function;
+  }
+
+}
diff --git a/src/applications/search/compiler/PhutilSearchStemmer.php b/src/applications/search/compiler/PhutilSearchStemmer.php
new file mode 100644
index 0000000000..3255336ff0
--- /dev/null
+++ b/src/applications/search/compiler/PhutilSearchStemmer.php
@@ -0,0 +1,74 @@
+<?php
+
+final class PhutilSearchStemmer
+  extends Phobject {
+
+  public function stemToken($token) {
+    $token = $this->normalizeToken($token);
+    return $this->applyStemmer($token);
+  }
+
+  public function stemCorpus($corpus) {
+    $corpus = $this->normalizeCorpus($corpus);
+    $tokens = preg_split('/[^a-zA-Z0-9\x7F-\xFF._]+/', $corpus);
+
+    $words = array();
+    foreach ($tokens as $key => $token) {
+      $token = trim($token, '._');
+
+      if (strlen($token) < 3) {
+        continue;
+      }
+
+      $words[$token] = $token;
+    }
+
+    $stems = array();
+    foreach ($words as $word) {
+      $stems[] = $this->applyStemmer($word);
+    }
+
+    return implode(' ', $stems);
+  }
+
+  private function normalizeToken($token) {
+    return phutil_utf8_strtolower($token);
+  }
+
+  private function normalizeCorpus($corpus) {
+    return phutil_utf8_strtolower($corpus);
+  }
+
+  /**
+   * @phutil-external-symbol class Porter
+   */
+  private function applyStemmer($normalized_token) {
+    // If the token has internal punctuation, handle it literally. This
+    // deals with things like domain names, Conduit API methods, and other
+    // sorts of informal tokens.
+    if (preg_match('/[._]/', $normalized_token)) {
+      return $normalized_token;
+    }
+
+    static $loaded;
+
+    if ($loaded === null) {
+      $root = dirname(phutil_get_library_root('phabricator'));
+      require_once $root.'/externals/porter-stemmer/src/Porter.php';
+      $loaded = true;
+    }
+
+
+    $stem = Porter::stem($normalized_token);
+
+    // If the stem is too short, it won't be a candidate for indexing. These
+    // tokens are also likely to be acronyms (like "DNS") rather than real
+    // English words.
+    if (strlen($stem) < 3) {
+      return $normalized_token;
+    }
+
+    return $stem;
+  }
+
+}
diff --git a/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php b/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php
new file mode 100644
index 0000000000..647386f52c
--- /dev/null
+++ b/src/applications/search/compiler/__tests__/PhutilSearchQueryCompilerTestCase.php
@@ -0,0 +1,220 @@
+<?php
+
+final class PhutilSearchQueryCompilerTestCase
+  extends PhutilTestCase {
+
+  public function testCompileQueries() {
+    $tests = array(
+      '' => null,
+      'cat dog' => '+"cat" +"dog"',
+      'cat -dog' => '+"cat" -"dog"',
+      'cat-dog' => '+"cat-dog"',
+
+      // If there are spaces after an operator, the operator applies to the
+      // next search term.
+      'cat - dog' => '+"cat" -"dog"',
+
+      // Double quotes serve as delimiters even if there is no whitespace
+      // between terms.
+      '"cat"dog' => '+"cat" +"dog"',
+
+      // This query is too long.
+      str_repeat('x', 2048) => false,
+
+      // Multiple operators are not permitted.
+      '++cat' => false,
+      '+-cat' => false,
+      '--cat' => false,
+
+      // Stray operators are not permitted.
+      '+' => false,
+      'cat +' => false,
+
+      // Double quotes must be paired.
+      '"' => false,
+      'cat "' => false,
+      '"cat' => false,
+      'A"' => false,
+      'A"B"' => '+"A" +"B"',
+    );
+
+    $this->assertCompileQueries($tests);
+
+    // Test that we compile queries correctly if the operators have been
+    // swapped to use "AND" by default.
+    $operator_tests = array(
+      'cat dog' => '"cat" "dog"',
+      'cat -dog' => '"cat" -"dog"',
+    );
+    $this->assertCompileQueries($operator_tests, ' |-><()~*:""&\'');
+
+
+    // Test that we compile queries correctly if the quote operators have
+    // been swapped to differ.
+    $quote_tests = array(
+      'cat dog' => '+[cat] +[dog]',
+      'cat -dog' => '+[cat] -[dog]',
+    );
+    $this->assertCompileQueries($quote_tests, '+ -><()~*:[]&|');
+
+  }
+
+  public function testCompileQueriesWithStemming() {
+    $stemming_tests = array(
+      'cat dog' => array(
+        null,
+        '+"cat" +"dog"',
+      ),
+      'cats dogs' => array(
+        null,
+        '+"cat" +"dog"',
+      ),
+      'cats "dogs"' => array(
+        '+"dogs"',
+        '+"cat"',
+      ),
+      '"blessed blade" of the windseeker' => array(
+        '+"blessed blade"',
+        '+"of" +"the" +"windseek"',
+      ),
+      'mailing users for mentions on tasks' => array(
+        null,
+        '+"mail" +"user" +"for" +"mention" +"on" +"task"',
+      ),
+    );
+
+    $stemmer = new PhutilSearchStemmer();
+    $this->assertCompileQueries($stemming_tests, null, $stemmer);
+  }
+
+  public function testCompileQueriesWithFunctions() {
+    $op_and = PhutilSearchQueryCompiler::OPERATOR_AND;
+    $op_sub = PhutilSearchQueryCompiler::OPERATOR_SUBSTRING;
+    $op_exact = PhutilSearchQueryCompiler::OPERATOR_EXACT;
+
+    $mao = "\xE7\x8C\xAB";
+
+    $function_tests = array(
+      'cat' => array(
+        array(null, $op_and, 'cat'),
+      ),
+      ':cat' => array(
+        array(null, $op_and, 'cat'),
+      ),
+      'title:cat' => array(
+        array('title', $op_and, 'cat'),
+      ),
+      'title:cat:dog' => array(
+        array('title', $op_and, 'cat:dog'),
+      ),
+      'title:~cat' => array(
+        array('title', $op_sub, 'cat'),
+      ),
+      'cat title:="Meow Meow"' => array(
+        array(null, $op_and, 'cat'),
+        array('title', $op_exact, 'Meow Meow'),
+      ),
+      'title:cat title:dog' => array(
+        array('title', $op_and, 'cat'),
+        array('title', $op_and, 'dog'),
+      ),
+      '~"core and seven years ag"' => array(
+        array(null, $op_sub, 'core and seven years ag'),
+      ),
+      $mao => array(
+        array(null, $op_sub, $mao),
+      ),
+      '+'.$mao => array(
+        array(null, $op_and, $mao),
+      ),
+      '~'.$mao => array(
+        array(null, $op_sub, $mao),
+      ),
+      '"'.$mao.'"' => array(
+        array(null, $op_and, $mao),
+      ),
+    );
+
+    $this->assertCompileFunctionQueries($function_tests);
+  }
+
+  private function assertCompileQueries(
+    array $tests,
+    $operators = null,
+    PhutilSearchStemmer $stemmer = null) {
+    foreach ($tests as $input => $expect) {
+      $caught = null;
+
+      $query = null;
+      $literal_query = null;
+      $stemmed_query = null;
+
+      try {
+        $compiler = new PhutilSearchQueryCompiler();
+
+        if ($operators !== null) {
+          $compiler->setOperators($operators);
+        }
+
+        if ($stemmer !== null) {
+          $compiler->setStemmer($stemmer);
+        }
+
+        $tokens = $compiler->newTokens($input);
+
+        if ($stemmer) {
+          $literal_query = $compiler->compileLiteralQuery($tokens);
+          $stemmed_query = $compiler->compileStemmedQuery($tokens);
+        } else {
+          $query = $compiler->compileQuery($tokens);
+        }
+      } catch (PhutilSearchQueryCompilerSyntaxException $ex) {
+        $caught = $ex;
+      }
+
+      if ($caught !== null) {
+        $query = false;
+        $literal_query = false;
+        $stemmed_query = false;
+      }
+
+      if (!$stemmer) {
+        $this->assertEqual(
+          $expect,
+          $query,
+          pht('Compilation of query: %s', $input));
+      } else {
+        $this->assertEqual(
+          $expect,
+          ($literal_query === false)
+            ? false
+            : array($literal_query, $stemmed_query),
+          pht('Stemmed compilation of query: %s', $input));
+      }
+    }
+  }
+
+  private function assertCompileFunctionQueries(array $tests) {
+    foreach ($tests as $input => $expect) {
+      $compiler = id(new PhutilSearchQueryCompiler())
+        ->setEnableFunctions(true);
+
+      $tokens = $compiler->newTokens($input);
+
+      $result = array();
+      foreach ($tokens as $token) {
+        $result[] = array(
+          $token->getFunction(),
+          $token->getOperator(),
+          $token->getValue(),
+        );
+      }
+
+      $this->assertEqual(
+        $expect,
+        $result,
+        pht('Function compilation of query: %s', $input));
+    }
+  }
+
+}
diff --git a/src/applications/search/compiler/__tests__/PhutilSearchStemmerTestCase.php b/src/applications/search/compiler/__tests__/PhutilSearchStemmerTestCase.php
new file mode 100644
index 0000000000..34154d9ca3
--- /dev/null
+++ b/src/applications/search/compiler/__tests__/PhutilSearchStemmerTestCase.php
@@ -0,0 +1,85 @@
+<?php
+
+final class PhutilSearchStemmerTestCase
+  extends PhutilTestCase {
+
+  public function testStemTokens() {
+    $tests = array(
+      // Various real-world cases collected from users before we implemented
+      // stemming.
+      'tokens' => 'token',
+      'panels' => 'panel',
+
+      'renames' => 'renam',
+      'rename' => 'renam',
+
+      'components' => 'compon',
+      'component' => 'compon',
+
+      'implementation' => 'implement',
+      'implements' => 'implement',
+      'implementing' => 'implement',
+      'implementer' => 'implement',
+
+      'deleting' => 'delet',
+      'deletion' => 'delet',
+      'delete' => 'delet',
+
+      'erratically' => 'errat',
+      'erratic' => 'errat',
+
+      // Stems should be normalized.
+      'DOG' => 'dog',
+
+      // If stemming would bring a token under 3 characters, it should not
+      // be stemmed.
+      'dns' => 'dns',
+      'nis' => 'nis',
+
+      // Complex tokens with internal punctuation should be left untouched;
+      // these are usually things like domain names, API calls, informal tags,
+      // etc.
+      'apples' => 'appl',
+      'bananas' => 'banana',
+      'apples_bananas' => 'apples_bananas',
+      'apples_bananas.apples_bananas' => 'apples_bananas.apples_bananas',
+    );
+
+    $stemmer = new PhutilSearchStemmer();
+    foreach ($tests as $input => $expect) {
+      $stem = $stemmer->stemToken($input);
+      $this->assertEqual(
+        $expect,
+        $stem,
+        pht('Token stem of "%s".', $input));
+    }
+  }
+
+  public function testStemDocuments() {
+    $tests = array(
+      'The wild boar meandered erratically.' =>
+        'the wild boar meander errat',
+      'Fool me onc, shame on you. Fool me twice, shame on me.' =>
+        'fool onc shame you twice',
+      'Fireball is a seventh-level spell which deals 2d16 points of damage '.
+      'in a 1-meter radius around a target.' =>
+        'firebal seventh level spell which deal 2d16 point damag meter '.
+        'radiu around target',
+      'apples-bananas' => 'appl banana',
+      'apples_bananas' => 'apples_bananas',
+      'apples.bananas' => 'apples.bananas',
+      'oddly-proportioned' => 'oddli proport',
+    );
+
+    $stemmer = new PhutilSearchStemmer();
+    foreach ($tests as $input => $expect) {
+      $stem = $stemmer->stemCorpus($input);
+      $this->assertEqual(
+        $expect,
+        $stem,
+        pht('Corpus stem of: %s', $input));
+    }
+  }
+
+
+}