Regular Expressions in Java  (Notes)

These are some notes about regular expressions in Java. This is not a lesson explaining every detail from the beginning.

Translating Perl -Idioms to Java

The following Perl -Script matches all the separated segments, where segments are separated by a colon, when the colon is not preceded by a question mark.

main.pl

while( "123:456?:789" =~ /(.*?)(?:(?<!\?):|$)/g )
{ print $1, "\n"; }

System.out

123
456?:789

In Java  the source string is translated to a matcher, the pattern to a pattern and the "=~" with the global flag "g" to a find operation of the matcher.

Main.java

public class Main
{ public static void main( final java.lang.String[] args )
  { final java.util.regex.Pattern p = 
    java.util.regex.Pattern.compile( "(.*?)(?:(?<!\\?):|$)" );
    final java.util.regex.Matcher m = p.matcher( "123:456?:789" );
    while( m.find() )java.lang.System.out.println( m.group( 1 )); }}

System.out

123
456?:789

Just replacing all occurrences of a pattern by another pattern can already be accomplished by simply using »java.lang.String#replaceAll«, which accepts a pattern as its first argument and allows groups to be referenced with a dollar sign and the group number in its second argument.

Cancellation of the Special Meaning of Special Characters

Some characters do not represent themselves within a regular expression but have a special meaning. The following program shows, how the static operation »java.util.regex.Pattern.quote« can be used to cancel such a special meaning.

Main.java

class String
{ final java.lang.String string;
  public String( final java.lang.String string )
  { this.string = string; }
  public java.lang.String[] split
  ( final java.lang.String text )
  { return this.string.split
    ( java.util.regex.Pattern.quote( text )); }}

public class Main
{ public static void main( final java.lang.String[] args )
  { final String text = new String( "alpha|beta" );
    final java.lang.String[] strings = text.split( "|" );
    for( java.lang.String string : strings ) 
    java.lang.System.out.println( string ); }}

System.out

alpha
beta

The following example shows how to cancel special meanings within the pattern and the replacement text of »java.lang.String#replaceAll«. However, this is not really needed here, because the operation »java.lang.String#replace« that does not use regular expressions could be used instead.

Main.java

class String
{ final java.lang.String string;
  public String( final java.lang.String string )
  { this.string = string; }
  public java.lang.String replaceAll
  ( final java.lang.String search, 
    final java.lang.String replace )
  { return this.string.replaceAll
    ( java.util.regex.Pattern.quote( search ),
      java.util.regex.Matcher.quoteReplacement( replace )); }}

public class Main
{ public static void main( final java.lang.String[] args )
  { java.lang.System.out.println
    ( new String( "alpha's" ).replaceAll( "'", "\\'" )); }}

System.out

alpha\'s

Negations

The following example shows how to match all texts, that do not  end in the text ».dwg«. (The pattern used is based on a comment by Ralf Ullrich .)

Main.java

class Test
{ final java.lang.String pattern;
  public Test( final java.lang.String pattern )
  { this.pattern = pattern; }
  public void text( final java.lang.String text )
  { java.lang.System.out.println
    ( text + " " +( text.matches( this.pattern ) ? "=" : "!" )+
      "~ " + this.pattern ); }}

public class Main
{ public static void main( final java.lang.String[] args )  
  { Test test = new Test( ".*(?<!\\.dwg)" );
    test.text( "abce.dwg" );
    test.text( "abce.dwx" );
    test.text( "abce.xwg" );
    test.text( "ab.cd.fg" ); 
    test.text( "abc.dwga" ); 
    test.text( "abcdefgh" ); }}

System.out

abce.dwg !~ .*(?<!\.dwg)
abce.dwx =~ .*(?<!\.dwg)
abce.xwg =~ .*(?<!\.dwg)
ab.cd.fg =~ .*(?<!\.dwg)
abc.dwga =~ .*(?<!\.dwg)
abcdefgh =~ .*(?<!\.dwg)