journalctl provides a bunch of different output messsages. This regex is searching for failed or accepted connection strings. The following groups are defined: ssh_id**:   The connection id. action**:   Can be 'Accepted' or 'Failed' username**:  The username ...

##My First regular expression

pattern to detect the CRC32 from a filename

This regex can read dates YYYY/MM/DD, dd/mm/yy, Month dd YYYY, YYYY Month dd where Month is the literal description of the month (jan, feb, mar, ..., dec). Use the ignore case flag. It has three capture groups. The regex doesn't handle the sequence logic, that has to be managed later on the capture...

1) (?ms)ID\s+.?^AC\s+(\w+);.?^OX\s+NCBI_TaxID=(\d+).?(?#how to optionally capture group ^FT\s+VAR_SEQ.?\/FTId=\w+\. ).?^\s{5}(.?)// 2) (?ms)(FT\s+VAR_SEQ.*?\/FTId=\w+\.) (1) can give AC, Tax_id and Sequence and these 3 fields are always present in an Entry, but VAR_SEQ field is optional. (2) gives ...

Example 0,2,3,5-6,9-23/2