Regular Expression

https://stackoverflow.com/questions/4105956/regex-does-not-contain-certain-characters

^[^<>]+$

The caret in the character class ([^) means match anything but, so this means, beginning of string, then one or more of anything except < and >, then the end of the string.

http://stackoverflow.com/questions/3012788/how-to-check-if-a-line-is-blank-using-regex

^\s*$

Explanation:

• ^ is the beginning of string anchor
• $ is the end of string anchor
• \s is the whitespace character class
• * is zero-or-more repetition of

In multiline mode, ^ and $ also match the beginning and end of the line.

http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#cache-jsr-107
JCache (JSR-107) annotations

https://www.digitalocean.com/community/tutorials/using-grep-regular-expressions-to-search-for-text-patterns-in-linux
Anchor Matches ^, $
Matching Any Character .
grep "..cept" GPL-3
match anything that has two characters and then the string "cept"

Bracket Expressions []
grep "t[wo]o" GPL-3
Repeat Pattern Zero or More Times *

Escaping Meta-Characters \
grep "^[A-Z].*\.$" GPL-3

http://www.thegeekstuff.com/2011/01/regular-expressions-in-grep-command/
http://stackoverflow.com/questions/6637882/how-can-i-use-grep-to-show-just-filenames-no-in-line-matches-on-linux

  -l, --files-with-matches
          Suppress  normal  output;  instead  print the name of each input
          file from which output would normally have  been  printed.   The
          scanning  will  stop  on  the  first match.  (-l is specified by
          POSIX.)

https://technet.microsoft.com/en-us/library/cc959336.aspx
DNS -

Characters

Supports RFC 1123, which permits "A" to "Z", "a" to "z", "0" to "9", and the hyphen (-).

https://www.quora.com/Why-are-underscores-not-allowed-in-DNS-host-names
The easy (historical) answer is: "_" is Shift "-". Because of the fact that domain names are not case sensitive "_" is the same as "-". By the way "_" is not a letter nor a number. It's a mechanic typewriter key ;-)

https://www.w3schools.com/jsref/jsref_regexp_wordchar.asp

The \w metacharacter is used to find a word character.

A word character is a character from a-z, A-Z, 0-9, including the _ (underscore) character.

Standard DNS doesn't allow underscore in hostname, but allows dash: -
\w matches underscore not dash