The below code works as expected as a standalone Groovy script (the if condition doesn't enter and so script value remains the same:

def script = '''mvn -q -Dexec.executable=echo -Dexec.args=\\${project.version} --non-recursive exec:exec'''
def index
if ((index = script.indexOf('release:prepare')) != -1) {
    script = script[0..<index-1] + ' -DpushChanges=false ' + script[index..-1]
}

But the exact same code in a custom vars function misbehaves. The if condition enters anyway with an index value of -1 resulting in a script value of mvn -q -Dexec.executable=echo -Dexec.args=\${project.version} --non-recursive exec:ex -DpushChanges=false c

If I pull the assignment out of the if condition, it works as expected:

def index = script.indexOf('release:prepare')
if (index != -1) {