Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/PlaceholderExpression.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/PlaceholderExpression.java	(revision 18758)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/PlaceholderExpression.java	(revision 18758)
@@ -0,0 +1,109 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.mappaint.mapcss;
+
+import static org.openstreetmap.josm.tools.I18n.tr;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.openstreetmap.josm.data.osm.Tag;
+import org.openstreetmap.josm.data.osm.Tagged;
+import org.openstreetmap.josm.gui.mappaint.Environment;
+import org.openstreetmap.josm.tools.CheckParameterUtil;
+import org.openstreetmap.josm.tools.Logging;
+
+/**
+ * Used for expressions that contain placeholders
+ * @since xxx
+ */
+public final class PlaceholderExpression implements Expression {
+    /**
+     * The regex used for pattern replacement
+     */
+    public static final Pattern PATTERN_PLACEHOLDER = Pattern.compile("\\{(\\d+)\\.(key|value|tag)}");
+    private final String placeholder;
+
+    /**
+     * Constructs a new {@link PlaceholderExpression}.
+     * @param placeholder The placeholder expression
+     */
+    public PlaceholderExpression(String placeholder) {
+        CheckParameterUtil.ensureParameterNotNull(placeholder);
+        this.placeholder = placeholder.intern();
+    }
+
+    @Override
+    public Object evaluate(Environment env) {
+        if (env.selector() == null) {
+            return placeholder;
+        }
+        return insertArguments(env.selector(), placeholder, env.osm);
+    }
+
+    /**
+     * Replaces occurrences of <code>{i.key}</code>, <code>{i.value}</code>, <code>{i.tag}</code> in {@code s} by the corresponding
+     * key/value/tag of the {@code index}-th {@link Condition} of {@code matchingSelector}.
+     *
+     * @param matchingSelector matching selector
+     * @param s                any string
+     * @param p                OSM primitive
+     * @return string with arguments inserted
+     */
+    public static String insertArguments(Selector matchingSelector, String s, Tagged p) {
+        if (s != null && matchingSelector instanceof Selector.ChildOrParentSelector) {
+            return insertArguments(((Selector.ChildOrParentSelector) matchingSelector).right, s, p);
+        } else if (s == null || !(matchingSelector instanceof Selector.GeneralSelector)) {
+            return s;
+        }
+        final Matcher m = PATTERN_PLACEHOLDER.matcher(s);
+        final StringBuffer sb = new StringBuffer();
+        while (m.find()) {
+            final String argument = determineArgument(matchingSelector,
+                    Integer.parseInt(m.group(1)), m.group(2), p);
+            try {
+                // Perform replacement with null-safe + regex-safe handling
+                m.appendReplacement(sb, String.valueOf(argument).replace("^(", "").replace(")$", ""));
+            } catch (IndexOutOfBoundsException | IllegalArgumentException e) {
+                Logging.log(Logging.LEVEL_ERROR, tr("Unable to replace argument {0} in {1}: {2}", argument, sb, e.getMessage()), e);
+            }
+        }
+        m.appendTail(sb);
+        return sb.toString();
+    }
+
+    /**
+     * Determines the {@code index}-th key/value/tag (depending on {@code type}) of the
+     * {@link org.openstreetmap.josm.gui.mappaint.mapcss.Selector}.
+     *
+     * @param matchingSelector matching selector
+     * @param index            index
+     * @param type             selector type ("key", "value" or "tag")
+     * @param p                OSM primitive
+     * @return argument value, can be {@code null}
+     */
+    private static String determineArgument(Selector matchingSelector, int index, String type, Tagged p) {
+        try {
+            final Condition c = matchingSelector.getConditions().get(index);
+            final Tag tag = c instanceof Condition.TagCondition
+                    ? ((Condition.TagCondition) c).asTag(p)
+                    : null;
+            if (tag == null) {
+                return null;
+            } else if ("key".equals(type)) {
+                return tag.getKey();
+            } else if ("value".equals(type)) {
+                return tag.getValue();
+            } else if ("tag".equals(type)) {
+                return tag.toString();
+            }
+        } catch (IndexOutOfBoundsException ioobe) {
+            Logging.debug(ioobe);
+        }
+        return null;
+    }
+
+    @Override
+    public String toString() {
+        return '<' + placeholder + '>';
+    }
+}