java - Parsing EL expression to get list of variable names -

i have expression, can can contain variables, operators, functions etc. examples:

${math:max(foo,bar)}

${input < 7}

${input}

${inputname}":"${inputvalue}-${mynamespace:func(parm1)}

i need able parse can list of variable names only. not want evaluate obtain variable names.

one way able create own elcontext, use custom variablemapper , build list of variables resolvevariable() method invoked. there way using javax.el or de.odysseus.el classes? thought might able build tree expression , walk tree don't see way.