regex digit between values code example

Example 1: regex ranges

/* character ranges */
regex = /[a-z]/; // matches all lowercase letters
regex = /[A-Z]/; // matches all uppercase letters
regex = /[e-l]/; // matches lowercase letters e to l (inclusive)
regex = /[F-P]/; // matches all uppercase letters F to P (inclusive)
regex = /[0-9]/; // matches all digits
regex = /[5-9]/; // matches any digit from 5 to 9 (inclusive)
regex = / [a-d1-7]/; // matches a letter between a and d and figures from 1 to 7, but not d1
regex = /[a-zA-Z]/; // matches all lowercase and uppercase letters
regex = /[^a-zA-Z]/; // matches non-letters

Example 2: regex match number

For matching an integer number of one single char/digit, specify:
[0-9]
But for matching any number of more than one char/digit; add "+":
[0-9]+
Example for matching hour with minutes of HH:MM format in a file:
grep "[0-9]\+\:[0-9]\+" file.txt