diff --git a/build.gradle b/build.gradle
index de9efde..0cb6160 100644
--- a/build.gradle
+++ b/build.gradle
@@ -2,6 +2,10 @@ plugins {
   id 'com.android.application' version '8.1.1'
 }
 
+dependencies {
+  testImplementation "junit:junit:4.13.2"
+}
+
 android {
   namespace 'juloo.keyboard2'
   compileSdk 34
@@ -21,6 +25,10 @@ android {
       res.srcDirs = ['res', 'build/generated-resources']
       assets.srcDirs = ['assets']
     }
+
+    test {
+      java.srcDirs = ['test']
+    }
   }
 
   signingConfigs {
@@ -84,9 +92,6 @@ android {
   }
 }
 
-dependencies {
-}
-
 tasks.register('buildKeyboardFont') {
   println "\nBuilding assets/special_font.ttf"
   mkdir "$buildDir"
diff --git a/srcs/juloo.keyboard2/Config.java b/srcs/juloo.keyboard2/Config.java
index f605381..174a7df 100644
--- a/srcs/juloo.keyboard2/Config.java
+++ b/srcs/juloo.keyboard2/Config.java
@@ -210,8 +210,7 @@ public final class Config
   KeyValue action_key()
   {
     // Update the name to avoid caching in KeyModifier
-    return (actionLabel == null) ? null :
-      KeyValue.getKeyByName("action").withSymbol(actionLabel);
+    return (actionLabel == null) ? null : KeyValue.makeActionKey(actionLabel);
   }
 
   /** Update the layout according to the configuration.
diff --git a/srcs/juloo.keyboard2/KeyValue.java b/srcs/juloo.keyboard2/KeyValue.java
index 2a329df..22f5a79 100644
--- a/srcs/juloo.keyboard2/KeyValue.java
+++ b/srcs/juloo.keyboard2/KeyValue.java
@@ -91,7 +91,8 @@ public final class KeyValue implements Comparable<KeyValue>
   {
     Char, String, Keyevent, Event, Compose_pending, Hangul_initial,
     Hangul_medial, Modifier, Editing, Placeholder,
-    Cursor_move // Value is encoded as a 16-bit integer
+    Cursor_move, // Value is encoded as a 16-bit integer.
+    Complex, // [_payload] is a [KeyValue.Complex], value is [Complex.Kind].
   }
 
   private static final int FLAGS_OFFSET = 19;
@@ -129,7 +130,13 @@ public final class KeyValue implements Comparable<KeyValue>
     check((((Kind.values().length - 1) << KIND_OFFSET) & ~KIND_BITS) == 0);
   }
 
-  private final String _symbol;
+  /**
+   * The symbol that is rendered on the keyboard as a [String].
+   * Except for keys of kind:
+   * - [String], this is also the string to output.
+   * - [Complex], this is an instance of [KeyValue.Complex].
+   */
+  private final Object _payload;
 
   /** This field encodes three things: Kind, flags and value. */
   private final int _code;
@@ -153,7 +160,9 @@ public final class KeyValue implements Comparable<KeyValue>
       When [getKind() == Kind.String], also the string to send. */
   public String getString()
   {
-    return _symbol;
+    if (getKind() == Kind.Complex)
+      return ((Complex)_payload).getSymbol();
+    return (String)_payload;
   }
 
   /** Defined only when [getKind() == Kind.Char]. */
@@ -211,25 +220,32 @@ public final class KeyValue implements Comparable<KeyValue>
     return (short)(_code & VALUE_BITS);
   }
 
+  /** Defined only when [getKind() == Kind.Complex]. */
+  public Complex getComplex()
+  {
+    return (Complex)_payload;
+  }
+
+  /** Defined only when [getKind() == Kind.Complex]. */
+  public Complex.Kind getComplexKind()
+  {
+    return Complex.Kind.values()[(_code & VALUE_BITS)];
+  }
+
   /* Update the char and the symbol. */
   public KeyValue withChar(char c)
   {
     return new KeyValue(String.valueOf(c), Kind.Char, c, getFlags());
   }
 
-  public KeyValue withSymbol(String s)
-  {
-    return new KeyValue(s, (_code & KIND_BITS), (_code & VALUE_BITS), getFlags());
-  }
-
   public KeyValue withKeyevent(int code)
   {
-    return new KeyValue(_symbol, Kind.Keyevent, code, getFlags());
+    return new KeyValue(getString(), Kind.Keyevent, code, getFlags());
   }
 
   public KeyValue withFlags(int f)
   {
-    return new KeyValue(_symbol, (_code & KIND_BITS), (_code & VALUE_BITS), f);
+    return new KeyValue(getString(), (_code & KIND_BITS), (_code & VALUE_BITS), f);
   }
 
   @Override
@@ -247,7 +263,9 @@ public final class KeyValue implements Comparable<KeyValue>
     d = _code - snd._code;
     if (d != 0)
       return d;
-    return _symbol.compareTo(snd._symbol);
+    if (getKind() == Kind.Complex)
+      return ((Complex)_payload).compareTo((Complex)snd._payload);
+    return ((String)_payload).compareTo((String)snd._payload);
   }
 
   /** Type-safe alternative to [equals]. */
@@ -255,24 +273,36 @@ public final class KeyValue implements Comparable<KeyValue>
   {
     if (snd == null)
       return false;
-    return _symbol.equals(snd._symbol) && _code == snd._code;
+    return _code == snd._code && _payload.equals(snd._payload);
   }
 
   @Override
   public int hashCode()
   {
-    return _symbol.hashCode() + _code;
+    return _payload.hashCode() + _code;
   }
 
-  public KeyValue(String s, int kind, int value, int flags)
+  public String toString()
   {
-    _symbol = s;
+    int value = _code & VALUE_BITS;
+    return "[KeyValue " + getKind().toString() + "+" + getFlags() + "+" + value + " \"" + getString() + "\"]";
+  }
+
+  private KeyValue(Object p, int kind, int value, int flags)
+  {
+    _payload = p;
     _code = (kind & KIND_BITS) | (flags & FLAGS_BITS) | (value & VALUE_BITS);
   }
 
-  public KeyValue(String s, Kind k, int v, int f)
+  public KeyValue(Complex p, Complex.Kind value, int flags)
   {
-    this(s, (k.ordinal() << KIND_OFFSET), v, f);
+    this((Object)p, (Kind.Complex.ordinal() << KIND_OFFSET), value.ordinal(),
+        flags);
+  }
+
+  public KeyValue(String p, Kind k, int v, int f)
+  {
+    this(p, (k.ordinal() << KIND_OFFSET), v, f);
   }
 
   private static KeyValue charKey(String symbol, char c, int flags)
@@ -397,6 +427,11 @@ public final class KeyValue implements Comparable<KeyValue>
     return KeyValue.makeCharKey((char)precomposed);
   }
 
+  public static KeyValue makeActionKey(String symbol)
+  {
+    return eventKey(symbol, Event.ACTION, FLAG_SMALLER_FONT);
+  }
+
   /** Make a key that types a string. A char key is returned for a string of
       length 1. */
   public static KeyValue makeStringKey(String str, int flags)
@@ -407,12 +442,36 @@ public final class KeyValue implements Comparable<KeyValue>
       return new KeyValue(str, Kind.String, 0, flags | FLAG_SMALLER_FONT);
   }
 
+  public static KeyValue makeStringKeyWithSymbol(String str, String symbol, int flags)
+  {
+    return new KeyValue(new Complex.StringWithSymbol(str, symbol),
+        Complex.Kind.StringWithSymbol, flags);
+  }
+
   /** Make a modifier key for passing to [KeyModifier]. */
   public static KeyValue makeInternalModifier(Modifier mod)
   {
     return new KeyValue("", Kind.Modifier, mod.ordinal(), 0);
   }
 
+  private static KeyValue parseKeyDefinition(String str)
+  {
+    if (str.length() < 2 || str.charAt(0) != ':')
+      return makeStringKey(str);
+    try
+    {
+      return KeyValueParser.parse(str);
+    }
+    catch (KeyValueParser.ParseError _e)
+    {
+      return makeStringKey(str);
+    }
+  }
+
+  /**
+   * Return a key by its name. If the given name doesn't correspond to a key
+   * defined in this function, it is passed to [parseStringKey] as a fallback.
+   */
   public static KeyValue getKeyByName(String name)
   {
     switch (name)
@@ -599,8 +658,8 @@ public final class KeyValue implements Comparable<KeyValue>
       case "ㅍ": return makeHangulInitial("ㅍ", 17);
       case "ㅎ": return makeHangulInitial("ㅎ", 18);
 
-      /* Fallback to a string key that types its name */
-      default: return makeStringKey(name);
+      /* The key is not one of the special ones. */
+      default: return parseKeyDefinition(name);
     }
   }
 
@@ -610,4 +669,49 @@ public final class KeyValue implements Comparable<KeyValue>
     if (!b)
       throw new RuntimeException("Assertion failure");
   }
+
+  public static abstract class Complex
+  {
+    public abstract String getSymbol();
+
+    /** [compareTo] can assume that [snd] is an instance of the same class. */
+    public abstract int compareTo(Complex snd);
+
+    public boolean equals(Object snd)
+    {
+      if (snd instanceof Complex)
+        return compareTo((Complex)snd) == 0;
+      return false;
+    }
+
+    /** [hashCode] will be called on this class. */
+
+    /** The kind is stored in the [value] field of the key. */
+    public static enum Kind
+    {
+      StringWithSymbol,
+    }
+
+    public static final class StringWithSymbol extends Complex
+    {
+      public final String str;
+      private final String _symbol;
+
+      public StringWithSymbol(String _str, String _sym)
+      {
+        str = _str;
+        _symbol = _sym;
+      }
+
+      public String getSymbol() { return _symbol; }
+
+      public int compareTo(Complex _snd)
+      {
+        StringWithSymbol snd = (StringWithSymbol)_snd;
+        int d = str.compareTo(snd.str);
+        if (d != 0) return d;
+        return _symbol.compareTo(snd._symbol);
+      }
+    }
+  };
 }
diff --git a/srcs/juloo.keyboard2/KeyValueParser.java b/srcs/juloo.keyboard2/KeyValueParser.java
new file mode 100644
index 0000000..178046e
--- /dev/null
+++ b/srcs/juloo.keyboard2/KeyValueParser.java
@@ -0,0 +1,150 @@
+package juloo.keyboard2;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/**
+Parse a key definition. The syntax for a key definition is:
+- [:(kind) (attributes):(payload)].
+- If [str] doesn't start with a [:] character, it is interpreted as an
+  arbitrary string key.
+
+[(kind)] specifies the kind of the key, it can be:
+- [str]: An arbitrary string key. The payload is the string to output when
+  typed and is quoted by single quotes ([']). The payload can contain single
+  quotes if they are escaped with a backslash ([\']).
+
+The [(attributes)] part is a space-separated list of attributes, all optional,
+of the form: [attrname='attrvalue'].
+
+Attributes can be:
+- [flags]: Add flags that change the behavior of the key.
+  Value is a coma separated list of:
+  - [dim]: Make the symbol dimmer on the keyboard.
+  - [small]: Make the symbol smaller on the keyboard.
+- [symbol]: Specify the symbol that is rendered on the keyboard.
+  It can contain single quotes if they are escaped: ([\']).
+
+Examples:
+- [:str flags=dim,small symbol='MyKey':'My arbitrary string'].
+- [:str:'My arbitrary string'].
+
+*/
+public final class KeyValueParser
+{
+  static Pattern START_PAT;
+  static Pattern ATTR_PAT;
+  static Pattern QUOTED_PAT;
+  static Pattern PAYLOAD_START_PAT;
+
+  static public KeyValue parse(String str) throws ParseError
+  {
+    String symbol = null;
+    int flags = 0;
+    init();
+    // Kind
+    Matcher m = START_PAT.matcher(str);
+    if (!m.lookingAt())
+      parseError("Expected kind, for example \":str ...\".", m);
+    String kind = m.group(1);
+    // Attributes
+    while (true)
+    {
+      if (!match(m, ATTR_PAT))
+        break;
+      String attr_name = m.group(1);
+      String attr_value = parseSingleQuotedString(m);
+      switch (attr_name)
+      {
+        case "flags":
+          flags = parseFlags(attr_value, m);
+          break;
+        case "symbol":
+          symbol = attr_value;
+          break;
+
+        default:
+          parseError("Unknown attribute "+attr_name, m);
+      }
+    }
+    // Payload
+    if (!match(m, PAYLOAD_START_PAT))
+      parseError("Unexpected character", m);
+    switch (kind)
+    {
+      case "str":
+        String payload = parseSingleQuotedString(m);
+        if (symbol == null)
+          return KeyValue.makeStringKey(payload, flags);
+        return KeyValue.makeStringKeyWithSymbol(payload, symbol, flags);
+
+      default: break;
+    }
+    parseError("Unknown kind '"+kind+"'", m, 1);
+    return null; // Unreachable
+  }
+
+  static String parseSingleQuotedString(Matcher m) throws ParseError
+  {
+    if (!match(m, QUOTED_PAT))
+      parseError("Expected quoted string", m);
+    return m.group(1).replace("\\'", "'");
+  }
+
+  static int parseFlags(String s, Matcher m) throws ParseError
+  {
+    int flags = 0;
+    for (String f : s.split(","))
+    {
+      switch (f)
+      {
+        case "dim": flags |= KeyValue.FLAG_SECONDARY; break;
+        case "small": flags |= KeyValue.FLAG_SMALLER_FONT; break;
+        default: parseError("Unknown flag "+f, m);
+      }
+    }
+    return flags;
+  }
+
+  static boolean match(Matcher m, Pattern pat)
+  {
+    try { m.region(m.end(), m.regionEnd()); } catch (Exception _e) {}
+    m.usePattern(pat);
+    return m.lookingAt();
+  }
+
+  static void init()
+  {
+    if (START_PAT != null)
+      return;
+    START_PAT = Pattern.compile(":(\\w+)");
+    ATTR_PAT = Pattern.compile("\\s*(\\w+)\\s*=");
+    QUOTED_PAT = Pattern.compile("'(([^'\\\\]+|\\\\')*)'");
+    PAYLOAD_START_PAT = Pattern.compile("\\s*:");
+  }
+
+  static void parseError(String msg, Matcher m) throws ParseError
+  {
+    parseError(msg, m, m.regionStart());
+  }
+
+  static void parseError(String msg, Matcher m, int i) throws ParseError
+  {
+    StringBuilder msg_ = new StringBuilder("Syntax error");
+    try
+    {
+      char c = m.group(0).charAt(0);
+      msg_.append(" at character '").append(c).append("'");
+    } catch (IllegalStateException _e) {}
+    msg_.append(" at position ");
+    msg_.append(i);
+    msg_.append(": ");
+    msg_.append(msg);
+    throw new ParseError(msg_.toString());
+  }
+
+  public static class ParseError extends Exception
+  {
+    public ParseError(String msg) { super(msg); }
+  };
+}
diff --git a/test/juloo.keyboard2/KeyValueParserTest.java b/test/juloo.keyboard2/KeyValueParserTest.java
new file mode 100644
index 0000000..900ae3d
--- /dev/null
+++ b/test/juloo.keyboard2/KeyValueParserTest.java
@@ -0,0 +1,54 @@
+package juloo.keyboard2;
+
+import juloo.keyboard2.KeyValue;
+import juloo.keyboard2.KeyValueParser;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class KeyValueParserTest
+{
+  public KeyValueParserTest() {}
+
+  @Test
+  public void parse() throws Exception
+  {
+    Utils.parse(":str:'Foo'", KeyValue.makeStringKey("Foo"));
+    Utils.parse(":str flags='dim':'Foo'", KeyValue.makeStringKey("Foo", KeyValue.FLAG_SECONDARY));
+    Utils.parse(":str symbol='Symbol':'Foo'", KeyValue.makeStringKeyWithSymbol("Foo", "Symbol", 0));
+    Utils.parse(":str symbol='Symbol' flags='dim':'Foo'", KeyValue.makeStringKeyWithSymbol("Foo", "Symbol", KeyValue.FLAG_SECONDARY));
+    Utils.parse(":str flags='dim,small':'Foo'", KeyValue.makeStringKey("Foo", KeyValue.FLAG_SECONDARY | KeyValue.FLAG_SMALLER_FONT));
+    Utils.parse(":str flags=',,':'Foo'", KeyValue.makeStringKey("Foo")); // Unintentional
+    Utils.expect_error(":unknown:Foo"); // Unknown kind
+    Utils.expect_error(":str:Foo"); // Unquoted string
+    Utils.expect_error(":str flags:'Foo'"); // Malformed flags
+    Utils.expect_error(":str flags=dim:'Foo'"); // Unquoted flags
+    Utils.expect_error(":str unknown='foo':'Foo'"); // Unknown flags
+    // Unterminated
+    Utils.expect_error(":str");
+    Utils.expect_error(":str ");
+    Utils.expect_error(":str flags");
+    Utils.expect_error(":str flags=");
+    Utils.expect_error(":str flags='");
+    Utils.expect_error(":str flags='' ");
+    Utils.expect_error(":str flags='':");
+    Utils.expect_error(":str flags='':'");
+  }
+
+  /** JUnit removes these functions from stacktraces. */
+  static class Utils
+  {
+    static void parse(String key_descr, KeyValue ref) throws Exception
+    {
+      assertEquals(ref, KeyValueParser.parse(key_descr));
+    }
+
+    static void expect_error(String key_descr)
+    {
+      try
+      {
+        fail("Expected failure but got " + KeyValueParser.parse(key_descr));
+      }
+      catch (KeyValueParser.ParseError e) {}
+    }
+  }
+}
diff --git a/test/juloo.keyboard2/KeyValueTest.java b/test/juloo.keyboard2/KeyValueTest.java
new file mode 100644
index 0000000..1fde92b
--- /dev/null
+++ b/test/juloo.keyboard2/KeyValueTest.java
@@ -0,0 +1,16 @@
+package juloo.keyboard2;
+
+import juloo.keyboard2.KeyValue;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class KeyValueTest
+{
+  public KeyValueTest() {}
+
+  @Test
+  public void equals()
+  {
+    assertEquals(KeyValue.makeStringKeyWithSymbol("Foo", "Symbol", 0), KeyValue.makeStringKeyWithSymbol("Foo", "Symbol", 0));
+  }
+}