Difference between String replace() and replaceAll()

Question

In java.lang.String, the replace method either takes a pair of char's or a pair of CharSequence's (of which String is a subclass, so it'll happily take a pair of String's). The replace method will replace all occurrences of a char or CharSequence. On the other hand, the first String arguments of replaceFirst and replaceAll are regular expressions (regex). Using the wrong function can lead to subtle bugs.

Q: What's the difference between the java.lang.String methods replace() and replaceAll(), other than that the latter uses regex.

A: Just the regex. They both replace all :)

http://docs.oracle.com/javase/8/docs/api/java/lang/String.html

PS:

There's also a replaceFirst() (which takes a regex)

Both replace() and replaceAll() replace all occurrences in the String.

Examples

I always find examples helpful to understand the differences.

replace()

Use replace() if you just want to replace some char with another char or some String with another String (actually CharSequence).

Example 1

Replace all occurrences of the character x with o.

String myString = "__x___x___x_x____xx_";

char oldChar = 'x';
char newChar = 'o';

String newString = myString.replace(oldChar, newChar);
// __o___o___o_o____oo_

Example 2

Replace all occurrences of the string fish with sheep.

String myString = "one fish, two fish, three fish";

String target = "fish";
String replacement = "sheep";

String newString = myString.replace(target, replacement);
// one sheep, two sheep, three sheep

replaceAll()

Use replaceAll() if you want to use a regular expression pattern.

Example 3

Replace any number with an x.

String myString = "__1_6____3__6_345____0";

String regex = "\\d";
String replacement = "x";

String newString = myString.replaceAll(regex, replacement); 
// __x_x____x__x_xxx____x

Example 4

Remove all whitespace.

String myString = "   Horse         Cow\n\n   \r Camel \t\t Sheep \n Goat        ";

String regex = "\\s";
String replacement = "";

String newString = myString.replaceAll(regex, replacement); 
// HorseCowCamelSheepGoat

See also

Documentation

• replace(char oldChar, char newChar)
• replace(CharSequence target, CharSequence replacement)
• replaceAll(String regex, String replacement)
• replaceFirst(String regex, String replacement)

Regular Expressions

• Tutorial
• List of patterns

The replace() method is overloaded to accept both a primitive char and a CharSequence as arguments.

Now as far as the performance is concerned, the replace() method is a bit faster than replaceAll() because the latter first compiles the regex pattern and then matches before finally replacing whereas the former simply matches for the provided argument and replaces.

Since we know the regex pattern matching is a bit more complex and consequently slower, then preferring replace() over replaceAll() is suggested whenever possible.

For example, for simple substitutions like you mentioned, it is better to use:

replace('.', '\\');

instead of:

replaceAll("\\.", "\\\\");

Note: the above conversion method arguments are system-dependent.

1. Both replace() and replaceAll() accepts two arguments and replaces all occurrences of the first substring(first argument) in a string with the second substring (second argument).
2. replace() accepts a pair of char or charsequence and replaceAll() accepts a pair of regex.

3. It is not true that replace() works faster than replaceAll() since both uses the same code in its implementation

   Pattern.compile(regex).matcher(this).replaceAll(replacement);

Now the question is when to use replace and when to use replaceAll(). When you want to replace a substring with another substring regardless of its place of occurrence in the string use replace(). But if you have some particular preference or condition like replace only those substrings at the beginning or end of a string use replaceAll(). Here are some examples to prove my point:

String str = new String("==qwerty==").replaceAll("^==", "?"); \\str: "?qwerty=="
String str = new String("==qwerty==").replaceAll("==$", "?"); \\str: "==qwerty?"
String str = new String("===qwerty==").replaceAll("(=)+", "?"); \\str: "?qwerty?"