Most used Perl regular expression

Includes
[0-9]	includes all digits
\d	includes all digits
\D	includes all that is not digits
[a-m]	includes all the character from a to m
[a-z]	includes all the character from a to z
[A-Z]	includes all upper character from A to Z
[^az]	includes all but not a nor z
\w	includes all character like [0-9a-zA-Z_]
\W	includes all that is not a character
\s	include a space, a tab, a new line or a return
\S	include the opposite of \s

Ex : [0-9a-zA-Z] will match any digit or letter from a to z uppercase or lowercase.

Pattern
*	0 or more
+	one or more
?	0 ore once
{n}	n times exactly
{n,}	at least n times
{,n}	at best n times
{n,m}	between n and m times

Ex : [0-9a-zA-Z]+ will match any number of letter or digit

tips : “\” neutralise a special character ex : “\.txt” neutralise the “.” Example of usage with grep :

grep -oP '/documents/[\w\s]+\.txt'
 -o excludes what doesn't match the regular expression
 -P use Perl regular expressions
To sum up this command ask grep to exclude all that doesn't start with /documents/ followed by one or more printable-characters-or-digits and ending with .txt

echo "<li class='pure-tree_link'><a href='/documents/Invoice_15_11_2020.pdf' target='_blank'>Invoice</a></li><li class='pure-tree_link'><a href='/documents/Report_15_01_2020.pdf' target='_blank'>Report</a></li><li class='pure-tree_link'><a href='/documents/flag_11dfa168ac8eb2958e38425728623c98.txt' target='_blank'>flag</a></li>      </ul>" | grep -oP '/documents/[\w\s]+\.txt'
/documents/flag_11dfa168ac8eb2958e38425728623c98.txt