Add "=expression" mask.

i.e. =perlin(0,x/2,y,z/2,0.05,3,0.8)<0.1

src/main/java/com/sk89q/worldedit/extension/factory/DefaultMaskParser.java

@@ -29,6 +29,7 @@ import com.sk89q.worldedit.extent.Extent;
 import com.sk89q.worldedit.function.mask.BiomeMask2D;
 import com.sk89q.worldedit.function.mask.BlockMask;
 import com.sk89q.worldedit.function.mask.ExistingBlockMask;
+import com.sk89q.worldedit.function.mask.ExpressionMask;
 import com.sk89q.worldedit.function.mask.Mask;
 import com.sk89q.worldedit.function.mask.MaskIntersection;
 import com.sk89q.worldedit.function.mask.Masks;
@@ -36,6 +37,7 @@ import com.sk89q.worldedit.function.mask.NoiseFilter;
 import com.sk89q.worldedit.function.mask.OffsetMask;
 import com.sk89q.worldedit.function.mask.RegionMask;
 import com.sk89q.worldedit.function.mask.SolidBlockMask;
+import com.sk89q.worldedit.internal.expression.ExpressionException;
 import com.sk89q.worldedit.internal.registry.InputParser;
 import com.sk89q.worldedit.math.noise.RandomNoise;
 import com.sk89q.worldedit.session.request.Request;
@@ -140,6 +142,13 @@ class DefaultMaskParser extends InputParser<Mask> {
                 int i = Integer.parseInt(component.substring(1));
                 return new NoiseFilter(new RandomNoise(), ((double) i) / 100);
 
+            case '=':
+                try {
+                    return new ExpressionMask(component.substring(1));
+                } catch (ExpressionException e) {
+                    throw new InputParseException("Invalid expression: " + e.getMessage());
+                }
+
             case '!':
                 if (component.length() > 1) {
                     return Masks.negate(getBlockMaskComponent(masks, component.substring(1), context));

src/main/java/com/sk89q/worldedit/function/mask/ExpressionMask.java

@@ -0,0 +1,77 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldedit.function.mask;
+
+import com.sk89q.worldedit.Vector;
+import com.sk89q.worldedit.internal.expression.Expression;
+import com.sk89q.worldedit.internal.expression.ExpressionException;
+import com.sk89q.worldedit.internal.expression.runtime.EvaluationException;
+
+import javax.annotation.Nullable;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * A mask that evaluates an expression.
+ *
+ * <p>Expressions are evaluated as {@code true} if they return a value
+ * greater than {@code 0}.</p>
+ */
+public class ExpressionMask extends AbstractMask {
+
+    private final Expression expression;
+
+    /**
+     * Create a new instance.
+     *
+     * @param expression the expression
+     * @throws ExpressionException thrown if there is an error with the expression
+     */
+    public ExpressionMask(String expression) throws ExpressionException {
+        checkNotNull(expression);
+        this.expression = Expression.compile(expression, "x", "y", "z");
+    }
+
+    /**
+     * Create a new instance.
+     *
+     * @param expression the expression
+     */
+    public ExpressionMask(Expression expression) {
+        checkNotNull(expression);
+        this.expression = expression;
+    }
+
+    @Override
+    public boolean test(Vector vector) {
+        try {
+            return expression.evaluate(vector.getX(), vector.getY(), vector.getZ()) > 0;
+        } catch (EvaluationException e) {
+            return false;
+        }
+    }
+
+    @Nullable
+    @Override
+    public Mask2D toMask2D() {
+        return new ExpressionMask2D(expression);
+    }
+
+}

src/main/java/com/sk89q/worldedit/function/mask/ExpressionMask2D.java

@@ -0,0 +1,63 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldedit.function.mask;
+
+import com.sk89q.worldedit.Vector2D;
+import com.sk89q.worldedit.internal.expression.Expression;
+import com.sk89q.worldedit.internal.expression.ExpressionException;
+import com.sk89q.worldedit.internal.expression.runtime.EvaluationException;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+public class ExpressionMask2D extends AbstractMask2D {
+
+    private final Expression expression;
+
+    /**
+     * Create a new instance.
+     *
+     * @param expression the expression
+     * @throws ExpressionException thrown if there is an error with the expression
+     */
+    public ExpressionMask2D(String expression) throws ExpressionException {
+        checkNotNull(expression);
+        this.expression = Expression.compile(expression, "x", "z");
+    }
+
+    /**
+     * Create a new instance.
+     *
+     * @param expression the expression
+     */
+    public ExpressionMask2D(Expression expression) {
+        checkNotNull(expression);
+        this.expression = expression;
+    }
+
+    @Override
+    public boolean test(Vector2D vector) {
+        try {
+            return expression.evaluate(vector.getX(), 0, vector.getZ()) > 0;
+        } catch (EvaluationException e) {
+            return false;
+        }
+    }
+
+}