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What is a regular expression?
Perl5 regular expressions






MatchResult example
Difference between matches() and contains()
Case sensitivity
Searching an InputStream


Package API reference (javadoc generated)


It is beyond the scope of this guide to give a detailed explanation of regular expressions to beginners. The OROMatcher TM package is geared toward programmers who are already familiar with regular expressions, having used them with other languages, and who now want to apply them in their Java programs. However, we shall make a small attempt to cover the basics and summarize the Perl5 syntax supported by the OROMatcher TM Perl5 classes. For a detailed exploration of regular expressions for both beginners and advanced users, we recommend the book Mastering Regular Expressions by Jeffrey Friedl published by O'Reilly & Associates.

What is a regular expression?

Part of this discussion is based on page 94 of "Compilers, Principles, Techniques, and Tools" by Aho, Sethi and Ullman

A regular expression is a pattern denoted by a sequence of symbols representing a state-machine or mini-program that is capable of matching particular sequences of characters. Regular expressions have their root in lexical analysis and tokenization where a set of lexemes had to be recognized before being passed on to a parser. Since then, regular expressions took a life of their own, appearing in such languages as AWK, TCL, and of course Perl, for all sorts of textual data extraction and manipulation purposes.

The most basic regular expression syntax consists of 4 operations. Let A and B each represent an alphabet (a set of characters) and s and t represent members of those alphabets.

Operation Representation Meaning
Union of A and B A|B s is such that s is in A or s is in B
Concatentation of A and B AB st are such that s is in A and t is in B
Kleene closure of A A* Zero or more concatenations of A
Positive closure of A A+ One or more concatenations of A

Using this notation you can define a regular expression for positive integers as follows:

digit +

Here digit represents the set of characters 0 - 9. A range of characters like this can be represented in most regular expression languages as [0-9]. Because this is such a common expression, some languages have a special character for it: \d .

Learning a regular expression language is quite simple once you've learned one, because most of the operations are the same. Only the notation changes.

Perl5 regular expressions

Here we summarize the syntax of Perl5 regular expressions, all of which is supported by the OROMatcher TM Perl5 classes. However, for a definitive reference, you should consult the perlre man page that accompanies the Perl5 distribution and also the book Programming Perl, 2nd Edition from O'Reilly & Associates. We need to point out here that for efficiency reasons the character set operator [...] is limited to work on only ASCII characters (Unicode characters 0 through 255). Other than that restriction, all Unicode characters should be useable in the package's regular expressions.

  • Alternatives separated by |
  • Quantified atoms
    Match at least n but not more than m times.
    Match at least n times.
    Match exactly n times.
    Match 0 or more times.
    Match 1 or more times.
    Match 0 or 1 times.
  • Atoms
    • regular expression within parentheses
    • a . matches everything except \n
    • a ^ is a null token matching the beginning of a string or line (i.e., the position right after a newline or right before the beginning of a string)
    • a $ is a null token matching the end of a string or line (i.e., the position right before a newline or right after the end of a string)
    • Character classes (e.g., [abcd]) and ranges (e.g. [a-z])
      • Special backslashed characters work within a character class (except for backreferences and boundaries).
      • \b is backspace inside a character class
    • Special backslashed characters
      null token matching a word boundary (\w on one side and \W on the other)
      null token matching a boundary that isn't a word boundary
      Match only at beginning of string
      Match only at end of string (or before newline at the end)
      carriage return
      digit [0-9]
      non-digit [^0-9]
      word character [0-9a-z_A-Z]
      a non-word character [^0-9a-z_A-Z]
      a whitespace character [ \t\n\r\f]
      a non-whitespace character [^ \t\n\r\f]
      hexadecimal representation of character
      matches the corresponding control character
      \nn or \nnn
      octal representation of character unless a backreference. a
      \1, \2, \3, etc.
      match whatever the first, second, third, etc. parenthesized group matched. This is called a backreference. If there is no corresponding group, the number is interpreted as an octal representation of a character.
      matches null character
      Any other backslashed character matches itself
  • Expressions within parentheses are matched as subpattern groups and saved for use by certain methods.

By default, a quantified subpattern is greedy . In other words it matches as many times as possible without causing the rest of the pattern not to match. To change the quantifiers to match the minimum number of times possible, without causing the rest of the pattern not to match, you may use a "?" right after the quantifier.

Match 0 or more times
Match 1 or more times
Match 0 or 1 time
Match exactly n times
Match at least n times
Match at least n but not more than m times

Perl5 extended regular expressions are fully supported.

An embedded comment causing text to be ignored.
Groups things like "()" but doesn't cause the group match to be saved.
A zero-width positive lookahead assertion. For example, \w+(?=\s) matches a word followed by whitespace, without including whitespace in the MatchResult.
A zero-width negative lookahead assertion. For example foo(?!bar) matches any occurrence of "foo" that isn't followed by "bar". Remember that this is a zero-width assertion, which means that a(?!b)d will match ad because a is followed by a character that is not b (the d) and a d follows the zero-width assertion.
One or more embedded pattern-match modifiers. i enables case insensitivity, m enables multiline treatment of the input, s enables single line treatment of the input, and x enables extended whitespace comments.
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