Globbing and Regex

January 9, 2021

This post is all about time saving tools. Globbing is for filename completion in shell and regex is for searching text.

Globbing

The asterisk means “zero or more characters” in the file glob. So this command returns all files ending in .md.

ls *.md

This command returns all files starting with A and ending with .md.

ls A*.md

The question mark means one unknown character in the file glob. So this command will return All.md but not About.md or A.md.

ls A??.md

Regex

^ means “at the beginning of the line” and $ means “at the end of the line”. So this commmand returns lines beginning with “moose”.

grep "^moose" file.txt

And this command returns lines ending with “mojo”.

grep "mojo$" file.txt

This command returns lines beginning with “moose” and ending with “mojo”. The .* means zero or more (*) of any character (.).

grep "^moose.*mojo$" file.txt

Regex Symbol	Utility
\	negate special-ness of character after
\s	whitespace (a space, tab, or newline)
?	one or more of the character that precedes it
.*	zero or more of any characters
vertical line	or
[]	within a range, for example ^[A-Z] to search line starting with capital
/i	makes regex before it case insensitive

grep and fgrep Come to Play

grep is a command line tool for searching text. grep never uses globbing. grep uses regex. However, with a command like this one:

grep file* README.md

the shell will do globbing before passing the command to grep. The shell will find filename.txt in the current directory (assuming it exists) and then grep will look for “filename.txt” in the README.md. However, adding quotes like this:

grep "file*" README.md

will prevent the shell from globbing and then file* will be interpretted as regex and grep will look for “fil” ending with zero or more “e” in README.md.

fgrep or grep -F on the other hand allows you to search for exactly what you type. For example, this command:

grep -F "$" file.txt

will return all the lines with the “$” character in file.txt.