Scriptrunner Script to validate Custom Number Field

Question

I try to validate a custom number field. I tried this insde the transistion from create->open

Everytime I try to create the issue I get an error:

"errorMessages": [ "Jira expression failed to parse: line 1, column 5:\n=, =>, ., ?., [, ?.[, (, *, /, %, +, -, <, <=, >, >=, ==, !=, &&, ||, ??, ?, ; or EOF expected, targetField encountered." ],

Maybe I have missed something with the following script?

def targetField = getCustomFieldValue("Target")
def val = targetField.getValue() as Number
def regexFormat = /^\d+$/
if(val ==~ regexFormat){
  targetField.clearError()
  return true
} else {
  targetField.setError("Required Target must be positive integer")
  return false
}