Checking a string for unwanted characters

As a developer, you frequently find yourself dealing with strings that are not created by your own code.

These will often be supplied by third party libraries, external systems, or even end users. Validating strings of unclear provenance is considered to be one of the hallmarks of defensive programming, and in most cases you will want to reject string input that does not meet your expectations.

A fairly common case is where you would only want to allow alphanumeric characters in an input string, so we'll use that as an example. In plain Java, the following two methods both serve the same purpose:

public static boolean isAlphanumeric(String s) {
    for (char c : s.toCharArray()) {
        if (!Character.isLetterOrDigit(c)) {
            return false;

    return true;
public static boolean isAlphanumeric(String s) {
    return s.matches("^[0­-9a­-zA­-Z]*$");

The first version converts the string to a character array, and then uses the Character class' static isLetterOrDigit method to determine whether the characters contained in the array are alphanumeric or not. This approach is predictable and readable, albeit a little bit verbose.

The second version uses a regular expression to achieve the same purpose. It is more concise, but can be somewhat enigmatic to developers with limited or no knowledge of regular expressions.

Guava introduces the CharMatcher class to deal with these types of situations. Our alphanumeric test, using Guava, would look as follows:

import static;

/* ... */

public static boolean isAlphanumeric(String s) {
    return javaLetterOrDigit().matchesAllOf(s);

The method body contains only one line, but there's actually a lot going on here, so let's break things down a little bit further.

If you take a look at the API of Guava's CharMatcher class, you'll notice that it implements the Predicate<Character> interface. If you would create a class that implements Predicate<Character> yourself, it could look something like this:


public class AlphanumericPredicate implements Predicate<Character> {
    public boolean apply(Character c) {
        return Character.isLetterOrDigit(c);

In Guava, as in a number of other programming languages and libraries that cater to a functional style of programming, a predicate is a construct that evaluates a given input to either true or false. In Guava's Predicate<T> interface, this is made evident by the presence of the sole boolean apply(T t) method. The CharMatcher class is built on this concept, and will evaluate a character or sequence of characters to check whether or not they match the criteria laid out by the used CharMatcher instance.

Guava currently provides the following predefined character matchers:

any()Matches any character.
none()Matches no characters.
javaDigit()Matches digits, according to the Java definition.
javaUpperCase()Matches any upper case character, according to Java's definition.
javaLowerCase()Matches any lower case character, according to Java's definition.
javaLetter()Matches any letter, according to Java's definition.
javaLetterOrDigit()Matches any letter or digit, according to Java's definition.
javaIsoControl()Matches any ISO control character, according to Java's definition.
ascii()Matches any character in the ASCII character set.
invisible()Matches characters that are not visible, according to the Unicode standard.
digit()Matches any digit, according to the Unicode specification.
whitespace()Matches any whitespace character, according to the Unicode specification.
breakingWhitespace()Matches any breaking whitespace character, according to the unicode specification.
singleWidth()Matches any single-­width character.

If you have read through the above table, you've undoubtedly noticed the amount of definition and specification involved in determining which characters belong to a certain category. Guava's approach, so far, has been to provide CharMatcher wrappers for a number of the character categories defined by Java, and you can consult the API of Java's Character class to get more information about these categories. On the other hand, Guava attempts to supply a number of CharMatcher instances that are in line with the current Unicode specification. For the nitty-gritty details, consult the CharMatcher API documentation.

Getting back to our example of checking a string for unwanted characters, the following CharMatcher methods provide the capabilities you need to check whether a given string's character usage meets your requirements:

  • boolean matchesNoneOf(CharSequence sequence)
    Returns true if none of the characters in the argument string match the CharMatcher instance.

  • boolean matchesAnyOf(CharSequence sequence)
    Returns true if at least one character in the argument string matches the CharMatcher instance.

  • boolean matchesAllOf(CharSequence sequence)
    Returns true if all of the characters in the argument string match the CharMatcher instance.

Finding and counting characters in a string

To help you find and count characters in a string, CharMatcher provides the following methods:

  • int indexIn(CharSequence sequence)
    Returns the index of the first character that matches the CharMatcher instance. Returns -­1 if no character matches.

  • int indexIn(CharSequence sequence, int start)
    Returns the index of the first character after the specified start position that matches the CharMatcher instance. Returns ­-1 if no character matches.

  • int lastIndexIn(CharSequence sequence)
    Returns the index of the last character that matches the CharMatcher instance. Returns ­-1 if no character matches.

  • int countIn(CharSequence sequence)
    Returns the number of characters that match the CharMatcher instance.

Using these methods, here's a simple console application called NonAsciiFinder that takes a string as an input argument. First, it prints out the total number of non­-ASCII characters contained in the string. Subsequently, it prints out the Unicode representation of each non-­ASCII character it encounters. Here's the code:


public class NonAsciiFinder {
    private static final CharMatcher NON_ASCII = CharMatcher.ascii().negate();

    public static void main(String[] args) {
        String input = args[0];
        int nonAsciiCount = NON_ASCII.countIn(input);

        echo("Non-ASCII characters found: %d", nonAsciiCount);

        if (nonAsciiCount > 0) {
            int position = -­1;
            char character = 0;

            while (position != NON_ASCII.lastIndexIn(input)) {
                position = NON_ASCII.indexIn(input, position + 1);
                character = input.charAt(position);
                echo("%s => \\u%04x", character, (int) character);

    private static void echo(String s, Object... args) {
        System.out.println(String.format(s, args));

Note in the above example how you can simply invert a CharMatcher by calling its negate method. Similarly the CharMatcher below matches all double­-width characters and is created by negating the predefined CharMatcher for single-width characters.

final static CharMatcher DOUBLE_WIDTH = CharMatcher.singleWidth().negate();

Running the NonAsciiFinder application produces the following output:

$> java NonAsciiFinder "Maître Corbeau, sur un arbre perché"
Non­-ASCII characters found: 2
î => \u00ee
é => \u00e9
$> java NonAsciiFinder "古池や蛙飛び込む水の音"
Non­ASCII characters found: 11
古 => \u53e4
池 => \u6c60
や => \u3084
蛙 => \u86d9
飛 => \u98db
び => \u3073
込 => \u8fbc
む => \u3080
水 => \u6c34
の => \u306e
音 => \u97f3

Removing unwanted characters from a string

The example Checking a string for unwanted characters, describes how to test and reject strings that don't meet certain criteria. Obviously, rejecting input outright is not always possible, and sometimes you just have to make do with what you receive. In these cases, a cautious developer will attempt to sanitize the provided strings to remove any characters that might trip up further processing.

To remove, trim, and replace unwanted characters, the weapon of choice will again be Guava's CharMatcher class.

Removing characters

The two CharMatcher methods of interest in this section are:

  • String retainFrom(CharSequence sequence)
    Returns a string containing all the characters that matched the CharMatcher instance.

  • String removeFrom(CharSequence sequence)
    Returns a string containing all the characters that did not match the CharMatcher instance.

As an example, we'll use CharMatcher.digit(), a predefined CharMatcher instance that, unsurprisingly, only matches digits.

String rock = "1, 2, 3 o'clock, 4 o'clock rock!";

CharMatcher.digit().retainFrom(rock); // "1234"
CharMatcher.digit().removeFrom(rock); // ", , o'clock, o'clock rock!"
CharMatcher.digit().negate().removeFrom(rock); // "1234"

The last line in this example illustrates that removeFrom is actually the inverse operation of retainFrom. Invoking retainFrom on a CharMatcher has the same effect as invoking removeFrom on a negated version of that CharMatcher.

Trimming leading and trailing characters

Removing leading and trailing characters is a very common operation, most frequently used to trim whitespace from strings. Guava's CharMatcher offers these trimming methods:

  • String trimLeadingFrom(CharSequence sequence)
    Removes all leading characters that match the CharMatcher instance.

  • String trimTrailingFrom(CharSequence sequence)
    Removes all trailing characters that match the CharMatcher instance.

  • String trimFrom(CharSequence sequence)
    Removes all leading and trailing characters that match the CharMatcher instance.

When used with CharMatcher.whitespace(), these methods will effectively take care of all your whitespace trimming needs:

CharMatcher.whitespace().trimFrom("   Too much space   "); // returns "Too much space"

Replacing characters

Often, applications will replace characters that are not allowed in a certain situation with a placeholder character. To replace characters in a string, CharMatcher's API provides the following methods:

  • String replaceFrom(CharSequence sequence, char replacement)
    Replaces all occurrences of characters that match the CharMatcher instance with the provided replacement character.

  • String replaceFrom(CharSequence sequence, CharSequence replacement) Replaces all occurrences of characters that match the CharMatcher instance with the provided replacement character sequence (string).

  • String collapseFrom(CharSequence sequence, char replacement)
    Replaces groups of consecutive characters that match the CharMatcher instance with a single instance of the provided replacement character.

  • String trimAndCollapseFrom(CharSequence sequence, char replacement)
    Behaves the same as collapseFrom, but matching groups at the start and the end are removed rather than replaced.

Let's look at an example that demonstrates how the behavior of these methods differs. Say that we're creating an application that lets the user specify output filenames. To sanitize the input provided by the user, we create a CharMatcher instance that is a combination of the predefined whitespace CharMatcher and a custom CharMatcher that specifies a set of characters that we would rather avoid in our filenames.

CharMatcher illegal = CharMatcher.whitespace().or(CharMatcher.anyOf("<>:|?*\"/\\"));

Now, if we invoke the discussed replacement methods as follows on a filename that is in dire need of cleanup:

String filename = "<A::12> first draft???";

System.out.println(illegal.replaceFrom(filename, '_'));
System.out.println(illegal.collapseFrom(filename, '_'));
System.out.println(illegal.trimAndCollapseFrom(filename, '_'));

We'll see the output below in our console.


Splitting a string into a list

To split strings, Guava introduces the Splitter class.

Why not use Java's splitting capabilities?

As a rule, Guava does not duplicate functionality that is readily available in Java. Why then do we need an additional Splitter class? Do the split methods in Java's String class not provide us with all the string splitting mechanics we'll ever need?

The easiest way to answer that question is with a couple of examples. First off, we'll deal with the following gunslinging duo:

String gunslingers = "Wyatt Earp+Doc Holliday";

To try and split up the legendary lawman and his dentist friend, we might try the following:

String[] result = gunslingers.split("+"); // wrong

At runtime, however, we are confronted with the following exception:

Exception in thread "main" java.util.regex.PatternSyntaxException:
Dangling meta character '+' near index 0

After an involuntary facepalm, we're quick to remember that String's split method takes a regular expression as an argument, and that the + character is used as a quantifier in regular expressions. The solution is then to escape the + character, or enclose it in a character class.

String[] result = gunslingers.split("\\+");
String[] result = gunslingers.split("[+]");

Having successfully resolved that issue, we move on to the three musketeers.

String musketeers = ",Porthos , Athos ,Aramis,";

The comma has no special meaning in regular expressions, so let's count the musketeers by applying the String.split() method and getting the length of the resulting array.


Which yields the following result in the console:


Four? Given the fact that the string contains a leading and a trailing comma, a result of five would have been within the realm of normal expectations, but four? As it turns out, the behavior of Java's split method is to preserve leading, but to discard trailing empty strings, so the actual contents of the array are ["", "Porthos ", " Athos ", "Aramis"].

Since we don't need any empty strings, leading nor trailing, let's filter them out with a loop:

for (String musketeer : musketeers.split(",")) {
    if (!musketeer.isEmpty()) {

This gives us the following output:


As you can see in the output above, the extra spaces before and after the comma separators have been preserved in the output. To get around that, we can trim off the unneeded spaces, which will finally yield the desired output:

for (String musketeer : musketeers.split(",")) {
    if(!musketeer.isEmpty()) {

(Alternatively, we could also adapt the regular expression to include whitespace surrounding the comma separators. However, keep in mind that leading spaces before the first entry or trailing spaces after the last entry would still be preserved.)

After reading through the examples above, we can't help but conclude that splitting strings with Java is mildly annoying at best.

Splitting strings with Guava

The best way to demonstrate how Guava turns splitting strings into a relatively pain­free experience, is to treat the same two strings again, but this time using Guava's Splitter class.

List<String> gunslingers = Splitter.on('+')
        .splitToList("Wyatt Earp+Doc Holliday");
List<String> musketeers = Splitter.on(",")
        .splitToList(",Porthos , Athos ,Aramis,");

As you can see in the code above, Splitter exposes a fluent API, and lets you create instances through a series of static factory methods:

  • static Splitter on(char separator)
    Lets you specify the separator as a character.

  • static Splitter on(String separator)
    Lets you specify the separator as a string.

  • static Splitter on(CharMatcher separatorMatcher)
    Lets you specify the separator as a Guava CharMatcher.

  • static Splitter on(Pattern separatorPattern)
    Lets you specify the separator as a Java regular expression Pattern.

  • static Splitter onPattern(String separatorPattern)
    Lets you specify the separator as a regular expression string.

In addition to these separator-­based factory methods, there's also a static Splitter fixedLength(int length) method to create Splitter instances that split strings into chunks of the specified length.

After the Splitter instance is created, a number of modifiers can be applied:

  • Splitter omitEmptyStrings()
    Instructs the Splitter to exclude empty strings from the results.

  • Splitter trimResults()
    Instructs the Splitter to trim results using the predefined whitespace CharMatcher.

  • Splitter trimResults(CharMatcher trimmer)
    Instructs the Splitter to trim results using the specified CharMatcher.

After creating (and optionally modifying) a Splitter, it can be invoked on a character sequence by invoking its split method, which will return an object of type Iterable<String>, or its splitToList method, which will return an (immutable) object of type List<String>.

You might wonder in which cases it would be beneficial to use the split method (which returns an Iterable) instead of the splitToList method (which returns the more commonly used List type). The short answer to that is: you probably want to use the split method only for processing very large strings. The slightly longer answer is that because the split method returns an Iterable, the split operations can be lazily evaluated (at iteration time), thus removing the need to keep the entire result of the split operation in memory.