Regular Expression Groups

Group Count

groupCount returns the number of groups in the pattern.

Accessing Groups

  • If there is one group, count is 1
  • To get the value, use group(1)
  • group(0) returns the entire matched value

Example

Pattern pattern = Pattern.compile("(\\d+)-(\\w+)");
Matcher matcher = pattern.matcher("123-abc");
if (matcher.find()) {
    matcher.group(0);  // "123-abc" (entire match)
    matcher.group(1);  // "123" (first group)
    matcher.group(2);  // "abc" (second group)
}

Lookahead and Lookbehind

Types

Syntax Name Description
(?<! Negative Lookbehind Asserts that what precedes does NOT match
(?<= Positive Lookbehind Asserts that what precedes DOES match
(?! Negative Lookahead Asserts that what follows does NOT match
(?= Positive Lookahead Asserts that what follows DOES match

Examples

(?<=@)\w+      # Match word after @
\w+(?=@)       # Match word before @
(?<!un)happy   # Match "happy" not preceded by "un"
happy(?!ness)  # Match "happy" not followed by "ness"

Backreferences (Repeat Groups)

Use \2 to reuse the second captured group.

Example

(\w+)\s+\1     # Match repeated words like "the the"

This matches a word, followed by whitespace, followed by the same word.

Password Validation Pattern

Complex Password Rule

^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*\W)(?!.*[^\u0000-\u007F])(?!.*\s).{8,50}$

Requirements

  • Contains at least one digit ((?=.*\d))
  • Contains at least one lowercase letter ((?=.*[a-z]))
  • Contains at least one uppercase letter ((?=.*[A-Z]))
  • Contains at least one special character ((?=.*\W))
  • No Unicode characters allowed ((?!.*[^\u0000-\u007F]))
  • No whitespace allowed ((?!.*\s))
  • Length between 8 and 50 characters (.{8,50})

Breakdown

Part Meaning
^ Start of string
(?=.*\d) Must contain digit
(?=.*[a-z]) Must contain lowercase
(?=.*[A-Z]) Must contain uppercase
(?=.*\W) Must contain special char
(?!.*[^\u0000-\u007F]) No non-ASCII chars
(?!.*\s) No whitespace
.{8,50} 8 to 50 characters
$ End of string