Refactor placeholder processing

Iterate over the characters of the template string and process placeholders from left to right instead of replacing placeholders in an arbitrary order. The main benefit to this approach is avoiding regex to locate the placeholders. Finding the placeholders with regex is trickier than it looks like at first glance especially when nested placeholder boundary characters have to be considered. For example '{' and '}' are reserved date pattern characters which might be used within a date placeholder in the future.

src/main/kotlin/de/idetools/filename_template/CreateFileWithFilenameTemplateAction.kt

@@ -14,6 +14,7 @@ import java.awt.event.InputEvent
 import java.time.LocalDateTime
 import java.time.ZoneOffset
 import java.util.function.Consumer
+import kotlin.math.max
 
 class CreateFileWithFilenameTemplateAction(
     text: String,
@@ -52,7 +53,47 @@ class CreateFileWithFilenameTemplateAction(
     }
 
     private fun processTemplate(template: String): String {
-        val epoch = LocalDateTime.now(ZoneOffset.UTC).toEpochSecond(ZoneOffset.UTC)
+        var placeholderLevel = 0
-        return template.replace("{EPOCH}", epoch.toString())
+        var placeholder = ""
+        var filename = ""
+
+        for (character in template) {
+            when (character) {
+                '{' -> {
+                    if (0 < placeholderLevel) {
+                        placeholder += character
+                    }
+
+                    placeholderLevel += 1
+                }
+                '}' -> {
+                    if (placeholderLevel == 1) {
+                        filename += processPlaceholder(placeholder)
+                        placeholder = ""
+                    }
+
+                    placeholderLevel = max(0, placeholderLevel - 1)
+
+                    if (0 < placeholderLevel) {
+                        placeholder += character
+                    }
+                }
+                else -> {
+                    if (0 < placeholderLevel) {
+                        placeholder += character
+                    } else {
+                        filename += character
+                    }
+                }
+            }
+        }
+
+        return filename
+    }
+
+    private fun processPlaceholder(placeholder: String): String =
+        when (placeholder) {
+            "EPOCH" -> LocalDateTime.now(ZoneOffset.UTC).toEpochSecond(ZoneOffset.UTC).toString()
+            else -> "{$placeholder}"
         }
 }