Regular expressions (Bash)
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)
Logical OR
Filter using regex
How can I filter stuff? E.g.:
12.5(diameter) → 12.5
Some tentative impressions:
- https://stackoverflow.com/questions/5624969/how-to-reference-captures-in-bash-regex-replacement
- https://stackoverflow.com/questions/13043344/search-and-replace-in-bash-using-regular-expressions
- https://www.google.com/search?q=bash%20parameter%20substitution%20with%20regular%20expressions
- https://stackoverflow.com/questions/19246966/regular-expression-in-bash-filter - Not really