how to get substring in soy template?

I have soy template in which I want only 28 character if the length is more than 30 characters? how to get that?

   {if $attributes.subAccountNumber.length < 30}
   {$attributes.subAccountNumber}  
    {else}
    {$attributes.subAccountNumber.substring(0,28)}                                                                                                              
    {/if}

But on compilation it throws exception,

Exception in thread "main" com.google.template.soy.base.SoySyntaxException: In file war/js/Application/Reports/ViewTemplates/ReportsViewTemplate.soy:518, template acti.workflow.reports.transcriptCard: Not all code is in Soy V2 syntax (found tag {$attributes.subAccountNumber.substring(0,28)} not in Soy V2 syntax).
at com.google.template.soy.base.SoySyntaxException.createWithoutMetaInfo(SoySyntaxException.java:52)
at com.google.template.soy.soytree.SoySyntaxExceptionUtils.createWithNode(SoySyntaxExceptionUtils.java:48)
at com.google.template.soy.sharedpasses.AssertSyntaxVersionV2Visitor.visitSoyNode(AssertSyntaxVersionV2Visitor.java:105)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visitPrintNode(AbstractSoyNodeVisitor.java:221)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visit(AbstractSoyNodeVisitor.java:85)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visit(AbstractSoyNodeVisitor.java:55)
at com.google.template.soy.basetree.AbstractNodeVisitor.visitChildren(AbstractNodeVisitor.java:59)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visitChildren(AbstractSoyNodeVisitor.java:126)
at com.google.template.soy.sharedpasses.AssertSyntaxVersionV2Visitor.visitSoyNode(AssertSyntaxVersionV2Visitor.java:112)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visitIfElseNode(AbstractSoyNodeVisitor.java:253)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visit(AbstractSoyNodeVisitor.java:95)
at com.google.template.soy.soytree.AbstractSoyNodeVisitor.visit(AbstractSoyNodeVisitor.java:55)

Solved

I don't believe that Closure templates allow you to call JavaScript functions on the objects. As much of an inconvenience as it is, you'll have evaluate {$attributes.subAccountNumber} before you pass it to the template (in Java or JS). I believe this is due to it remaining as a templating system and not a JS library.