Using Regular Expressions in JSP EL - regex

Question

In EL expressions, used in a jsp page, strings are taken literally. For example, in the following code snippet
<c:when test="${myvar == 'prefix.*'}">
test does not evaluate to true if the value of myvar is 'prefixxxxx.' Does anyone know if there is a way to have the string interpreted as a regex instead? Does EL have something similar to awk's tilde ~ operator?

Answer 1

While this special case can be handled with the JSTL fn:startsWith function, regular expressions in general seem like very likely tests. It's unfortunate that JSTL doesn't include a function for these.
On the bright side, it's pretty easy to write an EL function that does what you want. You need the function implementation, and a TLD to let your web application know where to find it. Put these together in a JAR and drop it into your WEB-INF/lib directory.
Here's an outline:
com/x/taglib/core/Regexp.java:
import java.util.regex.Pattern;
public class Regexp {
public static boolean matches(String pattern, CharSequence str) {
return Pattern.compile(pattern).matcher(str).matches();
}
}
META-INF/x-c.tld:
<taglib xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd" version="2.0">
<tlib-version>1.0</tlib-version>
<short-name>x-c</short-name>
<uri>http://dev.x.com/taglib/core/1.0</uri>
<function>
<description>Test whether a string matches a regular expression.</description>
<display-name>Matches</display-name>
<name>matches</name>
<function-class>com.x.taglib.core.Regexp</function-class>
<function-signature>boolean matches(java.lang.String, java.lang.CharSequence)</function-signature>
</function>
</taglib>
Sorry, I didn't test this particular function, but I hope it's enough to point you in the right direction.

Answer 2

Simply add the following to WEB-INF/tags.tld
<?xml version="1.0" encoding="ISO-8859-1" ?>
<taglib version="2.1"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd">
<display-name>Acme tags</display-name>
<short-name>custom</short-name>
<uri>http://www.acme.com.au</uri>
<function>
<name>matches</name>
<function-class>java.util.regex.Pattern</function-class>
<function-signature>
boolean matches(java.lang.String, java.lang.CharSequence)
</function-signature>
</function>
</taglib>
Then in your jsp
<%#taglib uri="http://www.acme.com.au" prefix="custom"%>
custom:matches('aaa.+', someVar) }
This will work exactly the same as Pattern.match

Answer 3

You can use JSTL functions like so -
<c:when test="${fn:startsWith(myVar, 'prefix')}">
Take a look: http://java.sun.com/products/jsp/jstl/1.1/docs/tlddocs/fn/tld-summary.html

Answer 4

for using Pattern.matches inside a jsp page in my case it was enough to call
java.util.regex.Pattern.matches(regexString,stringToCompare) because you can't import package in jsp