Class Optional

source code

     object --+        
              |        
  ParserElement --+    
                  |    
ParseElementEnhance --+
                      |
                     Optional

Optional matching of the given expression.

Parameters:

• expr - expression that must match zero or more times
• default (optional) - value to be returned if the optional expression is not found.

Example:

   # US postal code can be a 5-digit zip, plus optional 4-digit qualifier
   zip = Combine(Word(nums, exact=5) + Optional('-' + Word(nums, exact=4)))
   zip.runTests('''
       # traditional ZIP code
       12345
       
       # ZIP+4 form
       12101-0001
       
       # invalid ZIP
       98765-
       ''')

prints:

   # traditional ZIP code
   12345
   ['12345']

   # ZIP+4 form
   12101-0001
   ['12101-0001']

   # invalid ZIP
   98765-
        ^
   FAIL: Expected end of text (at char 5), (line:1, col:6)