Regular Expressions 101

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^(\d{6})(.csv$)
/
i
^ asserts position at start of the string
1st Capturing Group
(\d{6})
\d
matches a digit (equivalent to [0-9])
{6} matches the previous token exactly 6 times
2nd Capturing Group
(.csv$)
. matches any character (except for line terminators)
csv
matches the characters csv literally (case insensitive)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
i

Test String