[match character classes] #regex

a.sh

# Vowels
echo abcdefghijk | grep -e "[aeiou]"

# will find a,e,i

# Range
echo abcdefghijkAC04 | grep -e "[a-zA-D0-9]"


# Negation character ^ - don't match
echo abcdefghijkAC04 | grep -e "[^a-zA-D0-9]"
#AC04

# select with or (alternation)
echo "cat dog" | grep -e "[cat|fish]" # selects cat

### Using metacharacters

# select only characters (including _)
echo "P45SW0%$#_#2#4RD" | grep -e "\w" # selects P45SW0RD

# select only special characters
echo "P45SW0%$#_#2#4RD" | grep -e "\W" # selects %$#_##


# select all digits (-P flag enables perl regex)
echo "P45SW0%$#_#2#4RD" | grep -P "\d" # matches 

# select all except digits (-P flag enables perl regex)
echo "P45SW0%$#_#2#4RD" | grep -P "\D" # matches 

# select tabs, newlines
echo "some text with tab" | grep -P "\t" # matches tab  
echo "some text with tab" | grep -P "\n" # matches newline
echo "some text with tab" | grep -P "\s" # matches spaces
echo "some text with tab" | grep -P "\S" # matches sometextwiththab

# match square brackets
echo "[]" | grep -e "\[\]"