The below worked fine for me:

 

<xp:eventHandler event="onchange" submit="false">

<xp:this.script>

<xp:executeClientScript>

<xp:this.script><![CDATA[

var input = dojo.byId('#{id:inputText1}');

 

if(input.value == "example")

{

alert("ERROR");

input.focus();

}

                         ]]></xp:this.script>

</xp:executeClientScript>

</xp:this.script>

</xp:eventHandler>

 

 

however if this is for a form entry field and you have access to the ExtLib that comes from openNTF.org or passport advantage, I would suggest using the formTable control and a validator. Validators are already inbuilt, you can apply them to a form element and it won't allow form submission until the issue is resolved. The reason the form table is good is because it will display a yellow box on top of the control with a warning message and a list of all the validation fails and place a red X on top of each field that has failed. It adds a lot of functionality. 

 

e.g.  

 

 

<xe:formRow labelPosition="left" label="test">

<xp:inputText id="inputText1">

<xp:this.validators>

<xp:validateExpression message="Failed">

<xp:this.expression><![CDATA[#{javascript:

 

if(getComponent('inputText1').getValue() == "example")

{

return true;

}

else

{

return false;

}

 

}]]></xp:this.expression>

</xp:validateExpression>

</xp:this.validators>

</xp:inputText>

</xe:formRow>