textField variable not updating Follow

I'm trying to implement a tool window with a couple of text fields, but the variables bound to the fields are not updating after I type something.

class RegexTesterToolWindowFactory : ToolWindowFactory {
    override fun createToolWindowContent(project: Project, toolWindow: ToolWindow) {
        val regexTesterToolWindow = RegexTesterToolWindow(toolWindow)
        val contentFactory = ContentFactory.SERVICE.getInstance()
        val content = contentFactory.createContent(regexTesterToolWindow.content, "", false)
        toolWindow.contentManager.addContent(content)
    }
}

private val LOG = logger<RegexTesterToolWindow>()

class RegexTesterToolWindow(toolWindow: ToolWindow) {
    private var regex: String = ".*"
    private var testText: String = ""
    var content: JPanel

    init {
        content = panel {
            row("Regex:") {
                textField(::regex)
            }
            row {
                button("Run") {
                    val pattern = java.util.regex.Pattern.compile(regex)
                    val matcher = pattern.matcher(testText)
                    LOG.info(regex)
                    LOG.info(testText)
                }
            }
        }
    }

}

regex is always ".*" no matter what I type in the text field. What am I missing?