17.26 Select Lines from File Matching Regexp

20210304 We can select lines from a file that match a regular expression using grep with, for example, -P which will interpret the pattern as a Perl regular expression. For example, the select lines from any Rmd file in the current directory, that begin with two asterisks, then an 8 digit date, then two more asterisks and a space, we can use:

grep -P '^\*\*\d{8}\*\* ' *.Rmd


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