Meta-characters. There are special characters in regular expressions, some of them include: d matches any digit, equivalent to [0-9]; D matches any character that’s not a digit, equivalent to ...

Since version 3 (circa 2004), bash has a built-in regular expression comparison operator, represented by =~. A lot of scripting tricks that use grep or sed can now be handled by bash expressions ...

When working with regular expressions in a shell script the norm is to use grep or sed or some other external command/program. Since version 3 of bash (released in 2004) there is another option: ...