[Shell: regex (regular expression) patterns] #regex #sh #character #classes

regex.sh

##### REGEX CHARACTERS/GLOBS #####
. # match any one single character
* # match the previous character repeated 0+ times 
? # match the previous character repeated 0 or 1 time (extended pattern `-E`)
+ # match the previous character repeated 1+ times (extended pattern `-E`)
{<num>} # match the previous character repeated the number of times specificed in the braces (extended pattern `-E`)
[<chars>] # character range to match one time; check `ascii`
[^<chars>] # character range to not match one time; check `ascii`
^ # if first char of pattern, pattern must be at start of line to match
$ # if last char of pattern, pattern must be at end of line to match
| # logical 'or' (extended pattern `-E`)
\ # escape regex characters
() # not regex; used to group characters
grep -E '^[0-9]+' <file> # only matches lines starting with one or digits
grep -E '(red){3}' <file> # only matches lines with 3 or more instances of 'red'
grep -E 'colou?r' <file> # matches 'color' or 'colour'
grep -E 'grey|gray' <file> # matches 'grey' or 'gray'

##### CHARACTER CLASSES (POSIX) #####
# POSIX character classes generally require quoting or double brackets
[:alnum:] # matches alphabetic or numeric characters
[:alpha:] # matches alphabetic characters
[:blank:] # matches a space or a tab
[:digit:] # matches (decimal) digits
[:lower:] # matches lowercase alphabetic characters
[:space:] # matches whitespace characters (space and horizontal tab)
[:upper:] # matches uppercase alphabetic characters