Improve documentation for regular expressions.
Improve documentation for regular expressions.

Closes #1516

VirusTotal/yara,wxsBSD/yara,VirusTotal/yara,pombredanne/yara,pombredanne/yara,wxsBSD/yara,wxsBSD/yara,hillu/yara,VirusTotal/yara,hillu/yara,hillu/yara,VirusTotal/yara,hillu/yara,VirusTotal/yara,wxsBSD/yara,wxsBSD/yara,hillu/yara,pombredanne/yara,pombredanne/yara,pombredanne/yara