Regular Expressions: Examples

• Entire line.
      ^.*$
• Blank lines.
      ^$
• Whitespace on a line.
      \s+
• Whitespace across lines.
      (?n\s+)
• Whitespace that spans at least two lines.
      (?n\s*?\n\s*)       Note minimal matching `*?' quantifier
  
• IP address (not robust.)
      (?:\d{1,3}(?:\.\d{1,3}){3})
• Two characters US Postal state abbreviations (includes territories.)
      [ACDF-IK-PR-W][A-Z]
• Web addresses
      (?:http://)?www\.\S+
• Case insensitive double words across line breaks.
      (?i(?n<(\S+)\s+\1>))
• Upper case words with possible punctuation.
      <[A-Z][^a-z\s]*>
  

 References

An excellent book on the care and feeding of regular expressions is

"Mastering Regular Expressions"
Jeffrey E. F. Friedl
(c) 1997, O'Reilly & Associates
ISBN 1-56592-257-3