Regular Expressions

Regular Expression of Email:
string :rahul@gmail.com
exp.:([a-z])+@([a-z])+\.([a-z])+([a-z])

Passwords cannot contain sequences of 3 or more of the same character
(.)\1\1

• # the following expression finds double characters
  (.)\1
  

Brackets, Ranges and Negation[],- ^

Positioning (or Anchors)^,$,.

• The ^ (circumflex or caret) outside square brackets means look only at the beginning of the target string, for example,
• The $ (dollar) means look only at the end of the target string, for example, fox$ will
• The . (period) means any character(s) in this position,

Iteration MetaCharcters ?,* ,+,{n},{}

• The ? (question mark) matches the preceding character 0 or 1 times only

• The * (asterisk or star) matches the preceding character 0 or more times
• The + (plus) matches the previous character 1 or more times,
• {n} Matches the preceding character n times exactly
• {n,m} Matches the preceding character at least n times but not more than m times,

More metacharacters()|

Submatches, Groups and Backreferences

for testing and article Please follow
http://www.zytrax.com/tech/web/regex.htm