Regex pattern Kotlin

${month} is equal to (JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)

So the String """\d{2} ${month} \d{4}""" is actually expanded to

"""\d{2} (JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC) \d{4}"""

This a regex that captures a pair of numbers, followed by a space, then one of the values JAN, FEB... DEC, followed by another space and four more digits. So Strings like 04 APR 1234 match the regex.