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
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