def matchPreviousExpr(expr):
    """
    Helper to define an expression that is indirectly defined from
    the tokens matched in a previous expression, that is, it looks
    for a 'repeat' of a previous expression.  For example::
        first = Word(nums)
        second = matchPreviousExpr(first)
        matchExpr = first + ":" + second
    will match C{"1:1"}, but not C{"1:2"}.  Because this matches by
    expressions, will I{not} match the leading C{"1:1"} in C{"1:10"};
    the expressions are evaluated first, and then compared, so
    C{"1"} is compared with C{"10"}.
    Do I{not} use with packrat parsing enabled.
    """
    rep = Forward()
    e2 = expr.copy()
    rep <<= e2
    def copyTokenToRepeater(s,l,t):
        matchTokens = _flatten(t.asList())
        def mustMatchTheseTokens(s,l,t):
            theseTokens = _flatten(t.asList())
            if  theseTokens != matchTokens:
                raise ParseException("",0,"")
        rep.setParseAction( mustMatchTheseTokens, callDuringTry=True )
    expr.addParseAction(copyTokenToRepeater, callDuringTry=True)
    rep.setName('(prev) ' + _ustr(expr))
    return rep

    Helper to define an expression that is indirectly defined from
    the tokens matched in a previous expression, that is, it looks
    for a 'repeat' of a previous expression.  For example::
        first = Word(nums)
        second = matchPreviousExpr(first)
        matchExpr = first + ":" + second
    will match C{"1:1"}, but not C{"1:2"}.  Because this matches by
    expressions, will I{not} match the leading C{"1:1"} in C{"1:10"};
    the expressions are evaluated first, and then compared, so
    C{"1"} is compared with C{"10"}.
    Do I{not} use with packrat parsing enabled.
    
src/pkg_resources/_vendor/pyparsing.py