You can use text input's change event handler to add a condition block and match value of text box to set/unset required token.
<condition match="$value$!="*"">
Following is a run-anywhere example using Splunk's internal index
<form>
<label>Text box input validation</label>
<fieldset submitButton="false">
<input type="text" token="selLogLevel">
<label>Provide Log Level</label>
<change>
<condition match="$value$!="*"">
<set token="logLevelTok">$value$</set>
</condition>
<condition>
<unset token="logLevelTok"></unset>
</condition>
</change>
</input>
</fieldset>
<row>
<panel>
<table depends="$logLevelTok$">
<search>
<query>index=_internal sourcetype=splunkd log_level=$logLevelTok$
|stats count</query>
<earliest>-15m</earliest>
<latest>now</latest>
<sampleRatio>1</sampleRatio>
</search>
</table>
<html rejects="$logLevelTok$">
<div>
<p style="color:red;font-weight:bold;font-size:150%;text-align:left;">
Asterix * not allowed
</p>
</div>
</html>
</panel>
</row>
</form>
____________________________________________
| makeresults | eval message= "Happy Splunking!!!"
