diff --git a/src/main/kotlin/net/theevilreaper/dartpoet/DartFileBuilder.kt b/src/main/kotlin/net/theevilreaper/dartpoet/DartFileBuilder.kt
index 84e41165..4bbb3d6f 100644
--- a/src/main/kotlin/net/theevilreaper/dartpoet/DartFileBuilder.kt
+++ b/src/main/kotlin/net/theevilreaper/dartpoet/DartFileBuilder.kt
@@ -8,7 +8,7 @@ import net.theevilreaper.dartpoet.extension.ExtensionSpec
 import net.theevilreaper.dartpoet.directive.Directive
 import net.theevilreaper.dartpoet.property.DartPropertySpec
 import net.theevilreaper.dartpoet.util.DEFAULT_INDENT
-import java.lang.IllegalArgumentException
+import net.theevilreaper.dartpoet.util.isIndent
 
 class DartFileBuilder(
     val name: String
@@ -50,9 +50,7 @@ class DartFileBuilder(
     }
 
     fun indent(indent: String) = apply {
-        if (indent.trim().isEmpty()) {
-            throw IllegalArgumentException("The indent can't be empty")
-        }
+        check(isIndent(indent)) { "An indent can only contains only spaces" }
         this.indent = indent
     }
 
diff --git a/src/main/kotlin/net/theevilreaper/dartpoet/util/Constants.kt b/src/main/kotlin/net/theevilreaper/dartpoet/util/Constants.kt
index abd3d5e6..c0367f7b 100644
--- a/src/main/kotlin/net/theevilreaper/dartpoet/util/Constants.kt
+++ b/src/main/kotlin/net/theevilreaper/dartpoet/util/Constants.kt
@@ -30,8 +30,11 @@ internal val ALLOWED_FUNCTION_MODIFIERS = setOf(DartModifier.PUBLIC, DartModifie
 internal val ALLOWED_PROPERTY_MODIFIERS = setOf(DartModifier.PRIVATE, DartModifier.FINAL, DartModifier.LATE, DartModifier.STATIC, DartModifier.CONST)
 internal val ALLOWED_CLASS_CONST_MODIFIERS = setOf(DartModifier.STATIC, DartModifier.CONST)
 internal val ALLOWED_CONST_MODIFIERS = setOf(DartModifier.CONST)
+
+//RegEx
 private val namePattern: Regex = Regex("[a-z]+|([a-z]+)_+([a-z]+)")
 private val lowerCamelCase: Regex = Regex("[a-z]+[A-Z0-9]*[a-z0-9]*[A-Za-z0-9]*")
+private val indentPattern: Regex = Regex(" +")
 
 /**
  * Checks if a given set of [DartModifier] matches with a given set which contains the allowed [DartModifier].
@@ -67,5 +70,24 @@ fun isDartConventionFileName(fileName: String): Boolean {
  * @return true when the string has the format otherwise false
  */
 fun isInLowerCamelCase(input: String): Boolean {
-    return input.trim().isNotEmpty() && input.matches(lowerCamelCase)
-}
\ No newline at end of file
+    return testStringForPattern(input, lowerCamelCase)
+}
+
+/**
+ * Checks if a given [String] can be used as an indent.
+ * @param input the string to check
+ * @return true when the string only contains spaces otherwise false
+ */
+fun isIndent(input: String): Boolean {
+    return testStringForPattern(input, indentPattern)
+}
+
+/**
+ * Add base method for all the method which tests a string for a pattern.
+ * @param input the string to check
+ * @param pattern the pattern for the check
+ * @return true when the string matches with the pattern otherwise false
+ */
+private fun testStringForPattern(input: String, pattern: Regex): Boolean {
+    return input.isNotEmpty() && input.matches(pattern)
+}
diff --git a/src/test/kotlin/net/theevilreaper/dartpoet/DartFileTest.kt b/src/test/kotlin/net/theevilreaper/dartpoet/DartFileTest.kt
index f6ae3c09..d813c5a2 100644
--- a/src/test/kotlin/net/theevilreaper/dartpoet/DartFileTest.kt
+++ b/src/test/kotlin/net/theevilreaper/dartpoet/DartFileTest.kt
@@ -22,13 +22,13 @@ class DartFileTest {
     @Test
     fun `test indent set`() {
         assertThrows(
-            IllegalArgumentException::class.java,
+            IllegalStateException::class.java,
             { DartFile.builder("Test").indent("") },
             "The indent can't be empty"
         )
         assertThrows(
-            IllegalArgumentException::class.java,
-            { DartFile.builder("Test").indent { "" } },
+            IllegalStateException::class.java,
+            { DartFile.builder("Test").indent { " 123AB" } },
             "The indent can't be empty"
         )
     }
diff --git a/src/test/kotlin/net/theevilreaper/dartpoet/util/IndentTest.kt b/src/test/kotlin/net/theevilreaper/dartpoet/util/IndentTest.kt
new file mode 100644
index 00000000..21e29706
--- /dev/null
+++ b/src/test/kotlin/net/theevilreaper/dartpoet/util/IndentTest.kt
@@ -0,0 +1,33 @@
+package net.theevilreaper.dartpoet.util
+
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.MethodSource
+import java.util.stream.Stream
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class IndentTest {
+
+    companion object {
+
+        @JvmStatic
+        private fun indentTest() = Stream.of(
+            Arguments.of(" ", true),
+            Arguments.of("  ", true),
+            Arguments.of("", false),
+            Arguments.of(" a", false),
+            Arguments.of("123", false),
+        )
+    }
+
+    @ParameterizedTest
+    @MethodSource("indentTest")
+    fun `test indent pattern`(indent: String, expected: Boolean) {
+        if (expected) {
+            assertTrue { isIndent(indent) }
+        } else {
+            assertFalse { isIndent(indent) }
+        }
+    }
+}