How to check if a String contains another String in a case insensitive manner in Java?
Say I have two strings,
String s1 = "AbBaCca";
String s2 = "bac";
I want to perform a check returning that s2
is contained within s1
. I can do this with:
return s1.contains(s2);
I am pretty sure that contains()
is case sensitive, however I can't determine this for sure from reading the documentation. If it is then I suppose my best method would be something like:
return s1.toLowerCase().contains(s2.toLowerCase());
All this aside, is there another (possibly better) way to accomplish this without caring about case-sensitivity?
Yes, contains is case sensitive. You can use java.util.regex.Pattern with the CASE_INSENSITIVE flag for case insensitive matching:
Pattern.compile(Pattern.quote(wantedStr), Pattern.CASE_INSENSITIVE).matcher(source).find();
EDIT: If s2 contains regex special characters (of which there are many) it's important to quote it first. I've corrected my answer since it is the first one people will see, but vote up Matt Quail's since he pointed this out.
One problem with the answer by Dave L. is when s2 contains regex markup such as \d
, etc.
You want to call Pattern.quote() on s2:
Pattern.compile(Pattern.quote(s2), Pattern.CASE_INSENSITIVE).matcher(s1).find();
You can use
org.apache.commons.lang3.StringUtils.containsIgnoreCase("AbBaCca", "bac");
The Apache Commons library is very useful for this sort of thing. And this particular one may be better than regular expressions as regex is always expensive in terms of performance.
A Faster Implementation: Utilizing String.regionMatches()
Using regexp can be relatively slow. It (being slow) doesn't matter if you just want to check in one case. But if you have an array or a collection of thousands or hundreds of thousands of strings, things can get pretty slow.
The presented solution below doesn't use regular expressions nor toLowerCase()
(which is also slow because it creates another strings and just throws them away after the check).
The solution builds on the String.regionMatches() method which seems to be unknown. It checks if 2 String
regions match, but what's important is that it also has an overload with a handy ignoreCase
parameter.
public static boolean containsIgnoreCase(String src, String what) {
final int length = what.length();
if (length == 0)
return true; // Empty string is contained
final char firstLo = Character.toLowerCase(what.charAt(0));
final char firstUp = Character.toUpperCase(what.charAt(0));
for (int i = src.length() - length; i >= 0; i--) {
// Quick check before calling the more expensive regionMatches() method:
final char ch = src.charAt(i);
if (ch != firstLo && ch != firstUp)
continue;
if (src.regionMatches(true, i, what, 0, length))
return true;
}
return false;
}
Speed Analysis
This speed analysis does not mean to be rocket science, just a rough picture of how fast the different methods are.
I compare 5 methods.
- Our containsIgnoreCase() method.
- By converting both strings to lower-case and call
String.contains()
. - By converting source string to lower-case and call
String.contains()
with the pre-cached, lower-cased substring. This solution is already not as flexible because it tests a predefiend substring. - Using regular expression (the accepted answer
Pattern.compile().matcher().find()
...) - Using regular expression but with pre-created and cached
Pattern
. This solution is already not as flexible because it tests a predefined substring.
Results (by calling the method 10 million times):
- Our method: 670 ms
- 2x toLowerCase() and contains(): 2829 ms
- 1x toLowerCase() and contains() with cached substring: 2446 ms
- Regexp: 7180 ms
- Regexp with cached
Pattern
: 1845 ms
Results in a table:
RELATIVE SPEED 1/RELATIVE SPEED
METHOD EXEC TIME TO SLOWEST TO FASTEST (#1)
------------------------------------------------------------------------------
1. Using regionMatches() 670 ms 10.7x 1.0x
2. 2x lowercase+contains 2829 ms 2.5x 4.2x
3. 1x lowercase+contains cache 2446 ms 2.9x 3.7x
4. Regexp 7180 ms 1.0x 10.7x
5. Regexp+cached pattern 1845 ms 3.9x 2.8x
Our method is 4x faster compared to lowercasing and using contains()
, 10x faster compared to using regular expressions and also 3x faster even if the Pattern
is pre-cached (and losing flexibility of checking for an arbitrary substring).
Analysis Test Code
If you're interested how the analysis was performed, here is the complete runnable application:
import java.util.regex.Pattern;
public class ContainsAnalysis {
// Case 1 utilizing String.regionMatches()
public static boolean containsIgnoreCase(String src, String what) {
final int length = what.length();
if (length == 0)
return true; // Empty string is contained
final char firstLo = Character.toLowerCase(what.charAt(0));
final char firstUp = Character.toUpperCase(what.charAt(0));
for (int i = src.length() - length; i >= 0; i--) {
// Quick check before calling the more expensive regionMatches()
// method:
final char ch = src.charAt(i);
if (ch != firstLo && ch != firstUp)
continue;
if (src.regionMatches(true, i, what, 0, length))
return true;
}
return false;
}
// Case 2 with 2x toLowerCase() and contains()
public static boolean containsConverting(String src, String what) {
return src.toLowerCase().contains(what.toLowerCase());
}
// The cached substring for case 3
private static final String S = "i am".toLowerCase();
// Case 3 with pre-cached substring and 1x toLowerCase() and contains()
public static boolean containsConverting(String src) {
return src.toLowerCase().contains(S);
}
// Case 4 with regexp
public static boolean containsIgnoreCaseRegexp(String src, String what) {
return Pattern.compile(Pattern.quote(what), Pattern.CASE_INSENSITIVE)
.matcher(src).find();
}
// The cached pattern for case 5
private static final Pattern P = Pattern.compile(
Pattern.quote("i am"), Pattern.CASE_INSENSITIVE);
// Case 5 with pre-cached Pattern
public static boolean containsIgnoreCaseRegexp(String src) {
return P.matcher(src).find();
}
// Main method: perfroms speed analysis on different contains methods
// (case ignored)
public static void main(String[] args) throws Exception {
final String src = "Hi, I am Adam";
final String what = "i am";
long start, end;
final int N = 10_000_000;
start = System.nanoTime();
for (int i = 0; i < N; i++)
containsIgnoreCase(src, what);
end = System.nanoTime();
System.out.println("Case 1 took " + ((end - start) / 1000000) + "ms");
start = System.nanoTime();
for (int i = 0; i < N; i++)
containsConverting(src, what);
end = System.nanoTime();
System.out.println("Case 2 took " + ((end - start) / 1000000) + "ms");
start = System.nanoTime();
for (int i = 0; i < N; i++)
containsConverting(src);
end = System.nanoTime();
System.out.println("Case 3 took " + ((end - start) / 1000000) + "ms");
start = System.nanoTime();
for (int i = 0; i < N; i++)
containsIgnoreCaseRegexp(src, what);
end = System.nanoTime();
System.out.println("Case 4 took " + ((end - start) / 1000000) + "ms");
start = System.nanoTime();
for (int i = 0; i < N; i++)
containsIgnoreCaseRegexp(src);
end = System.nanoTime();
System.out.println("Case 5 took " + ((end - start) / 1000000) + "ms");
}
}