The expresiion detects start of multilane log message.

output from zsh configure

Chase Bank Credit Card Transaction Parser - Matches on transaction Date, Description, Amount, and (if it's a Chase Amazon card) the Amazon Order Numbers and total Amazon Rewards Points!

the whole word and nothing but the word

Matches all alphanumeric (\w), underscore and dash (\-) that is included in square brackets ([]). The result gives two groups that can be used with $1 and $2.