diff --git a/src/main/kotlin/uk/co/neviyn/booru/Entity.kt b/src/main/kotlin/uk/co/neviyn/booru/Entity.kt
index 047de0c..d80f443 100644
--- a/src/main/kotlin/uk/co/neviyn/booru/Entity.kt
+++ b/src/main/kotlin/uk/co/neviyn/booru/Entity.kt
@@ -8,6 +8,7 @@ import javax.persistence.JoinColumn
 import javax.persistence.JoinTable
 import javax.persistence.ManyToMany
 import javax.persistence.ManyToOne
+import javax.validation.constraints.Pattern
 
 @Entity
 open class User(
@@ -56,6 +57,7 @@ open class Image(
 @Entity
 open class Tag(
     @Column(unique = true)
+    @Pattern(regexp = "[a-zA-Z0-9_]*") // Only allow alphanumeric and underscores
     open var tag: String = "",
     @Id open var id: Long = -1
 )
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