Using Regular Expressions in JSP EL

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?

Dishpan

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.

Marsden

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

Mesics

Clever solution! But would it be possible to return the match result. I tried something with the java.util.regex.Matcher class but I don't know to make it work with that tld-syntax... – Theory

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

Adp

This doesn't answer op's (and my) question: "Does anyone know if there is a way to have the string interpreted as a regex instead?" – Allanite

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

Dunnock

Noted should be that this wasn't possible until EL 2.2 was released and the question was asked more than one year before that. – Entente

@Entente you are perfectly right, i just shared for everyone are searching now :) – Dunnock