Regular expressions

A regular expression is a String that describes a pattern; the pattern may be applied to another string, to determine if it matches the pattern, or to find and/or replace parts of that other string.

While regular expressions are an advanced topic, the beginner should be aware of two important facts about them:

• Several methods in the String class take a regular expression as an argument. These are:

  boolean matches(String regex)

  Tests whether this string matches the regex pattern.

  
String replaceFirst(String regex, String replacement)

  Replaces the first substring that matches the regex pattern with the replacement String.

  
String replaceAll(String regex, String replacement)

  Replaces every substring that matches the regex pattern with the replacement String.

  
public String[] split(String regex)

  Strings matched by the regex pattern act as separators; the substrings thus separated are returned in an array.

  
public String[] split(String regex, int limit)

  Returns an array of not more than limit substrings.

• Many punctuation marks have special meanings in regular expressions; letters, digits, and spaces do not. Hence, methods which take a regular expression as an argument may safely be used with punctuation-free strings as patterns, even without knowing regular expression syntax.

Regular expressions have quite a complex syntax that is outside the scope of these pages. Details are given in the Java API for java.util.regex.Pattern.

Regular expressions are very powerful, and should be a part of every programmer's toolbox.

However, regular expressions that use characters with special meanings can be extraordinarily inefficient. Any use of such regular expressions should be tested to determine not only their results, but also the sequence of operations performed in order to return those results. Often, very minor changes in a regular expression can have major consequences for efficiency.