Java String replaceFirst()

The syntax of the replaceFirst() method is:

 string.replaceFirst(String regex, String replacement)

Here, string is an object of the String class.

replaceFirst() Parameters

The replaceFirst() method takes two parameters.

• regex - a regex (can be a typical string) that is to be replaced
• replacement - the first matching substring is replaced with this string

replaceFirst() Return Value

• The replaceFirst() method returns a new string where the first occurrence of the matching substring is replaced with the replacement string.

Example 1: Java String replaceFirst()

 class Main { public static void main(String[] args) { String str1 = "aabbaaac"; String str2 = "Learn223Java55@"; // regex for sequence of digits String regex = "\\d+"; // the first occurrence of "aa" is replaced with "zz" System.out.println(str1.replaceFirst("aa", "zz")); // zzbbaaac // replace the first sequence of digits with a whitespace System.out.println(str2.replaceFirst(regex, " ")); // Learn Java55@ } }

In the above example, "\\d+" is a regular expression that matches a sequence of digits.

Escaping Characters in replaceFirst()

The replaceFirst() method can take a regex or a typical string as the first argument. It is because a typical string in itself is a regex.

In regex, there are characters that have special meaning. These metacharacters are:

 \ ^ $ . | ? * + {} [] ()

If you need to match substring containing these metacharacters, you can escape these characters using \.

 // Program to the first + character class Main { public static void main(String[] args) { String str = "a+a-++b"; // replace the first "+" with "#" System.out.println(str.replaceFirst("\\+", "#")); // a#a-++b } }

Also Read:

• Java String replaceAll() - replaces each substring that matches the regex
• Java String replace() - replaces each matching occurence of a character in the string