string operations inside roslauch file

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I have an argument defined inside the launch file as shown below-

<arg name="velocity" default="/mx230/motion/vel"/>

I want to define another argument, which is basically the string inside the first two forward slashes. For the example above, it should be similar to following-

<arg name="name" default="mx230"/>

I am wondering if there is an automatic way to find out this string. I can use the following python code inside my node but I feel that this simple operation could be possible inside launch file-

# get the name as the first word between two leftmost slash characters
def get_sensor_name(topic):
    import re
    all_slash = [m.start() for m in re.finditer('/', topic)]
    name = topic[all_slash[0] + 1 : all_slash[1]]
    return name

How to define arg name from arg velocity inside launch file?