Regular expressions are used to check strings for a certain criteria. Be aware that it is up to the software developer to support regular expression, but also to create them or even deviate from the following regular expressions.
^ Matches the beginning of the line
$ Matches the end of the line
. Matches any single character
* Will match zero or more occurrences of the previous character
[ ] Matches all the characters inside the [ ]
/./ Will match any line that contains at least one character
/../ Will match any line that contains at least two characters
/^#/ Will match any line that begins with a '#'
/^$/ Will match all blank lines
/}^/ Will match any lines that ends with '}' (no spaces)
/} *^/ Will match any line ending with '}' followed by zero or more spaces
/[abc]/ Will match any line that contains a lowercase 'a', 'b', or 'c'
/^[abc]/ Will match any line that begins with an 'a', 'b', or 'c'
[a-x]*
[:alnum:] Alphanumeric [a-z A-Z 0-9]
[:alpha:] Alphabetic [a-z A-Z]
[:blank:] Spaces or tabs
[:cntrl:] Any control characters
[:digit:] Numeric digits [0-9]
[:graph:] Any visible characters (no whitespace)
[:lower:] Lower-case [a-z]
[:print:] Non-control characters
[:punct:] Punctuation characters
[:space:] Whitespace
[:upper:] Upper-case [A-Z]
[:xdigit:] hex digits [0-9 a-f A-F]