Regular expressions (Bash)
Versie door Jeroen Strompf (overleg | bijdragen) op 18 okt 2022 om 13:04 (→Match ranges of numbers or letters)
Regular expressions, or regex to make it sound more mysterious, are the black magic of programming. It's like doing mathematics with text. Or slightly more specific: advanced pattern matching.
Places where you find regex:
- In
grep
for filtering output of a command - In comparisons, through use of the operator
=~
- Probably lots of other places.
I have the impression that regular expressions (regex) in Bash may not be the same as in MySQL, hence some more details in this article.
Match a substring
Probably the easiest case - No special characters or whatever needed:
i="blub"; [[ $i =~ blubber ]] && echo "i contains the substring 'blubber' " # False i="blub"; [[ $i =~ blub ]] && echo "i contains the substring 'blub' " # True
Match a single digit
[]
denotes single character-comparison, meaning that the comparison is true as soon as the string contains one of the characters indicated within []
.
[[ $i =~ [2] ]] && echo "i contains '2'" [[ $i =~ [12] ]] && echo "i contains '1' and/or '2'"
Match ranges of numbers or letters
i="blub"; [[ $i =~ [0-9] ]] && echo "i contains a number" # False i="blu1"; [[ $i =~ [0-9] ]] && echo "i contains a number" # True i="1111"; [[ $i =~ [0-9] ]] && echo "i contains a number" # True i="blub"; [[ $i =~ [A-Z] ]] && echo "i contains at least one uppercase letter" # False i="BLUB"; [[ $i =~ [A-Z] ]] && echo "i contains at least one uppercase letter" # True i="blub"; [[ $i =~ [a-z] ]] && echo "i contains at least one lowercase letter" # True i="blub"; [[ $i =~ [a-zA-Z] ]] && echo "i contains at least one letter" # True
Using grub:
echo 12345 | grep [0-9] 12345
Sequences
^
: Beginning of the string$
: End of the string
i="BLuB"; [[ $i =~ ^[A-Z]+$ ]] && echo "i contains only capital letters" # False i="BLUB"; [[ $i =~ ^[A-Z]+$ ]] && echo "i contains only capital letters" # True
Capture group
Really cool stuff: Substring extraction (Bash)