pattern

compact pattern pattern=re.compile(r"(CM|CD|D?C{0,3})")

re.search(pattern, 'M', re.VERBOSE)

• 1. basic pattern includes three elements

     1. atomic string:

     2. only consists of characters could be occurred in context string

     3. position sign:

     4. point the position of atomic pattern string in the context string

     5. repeat sign:

     6. the times that an atomic string will repeat

  2. unit pattern

  3. patterns put into to brace ()

  4. exclusive pattern: a series of basic patterns separated by exclusive sign | in a pair of braces

verbose pattern

> pattern = """

^ # beginning of string

M{0,4} # thousands - 0 to 4 M's

(CM|CD|D?C{0,3}) # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 C's),

# or 500-800 (D, followed by 0 to 3 C's)

(XC|XL|L?X{0,3}) # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 X's),

# or 50-80 (L, followed by 0 to 3 X's)

(IX|IV|V?I{0,3}) # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 I's),

# or 5-8 (V, followed by 0 to 3 I's)

$ # end of string

"""

re.search(pattern, 'M', re.VERBOSE)

or

phonePattern = re.compile(r'''

# don't match beginning of string, number can start anywhere

(\d{3}) # area code is 3 digits (e.g. '800')

\D* # optional separator is any number of non-digits

(\d{3}) # trunk is 3 digits (e.g. '555')

\D* # optional separator

(\d{4}) # rest of number is 4 digits (e.g. '1212')

\D* # optional separator

(\d*) # extension is optional and can be any number of digits

$ # end of string

''', re.VERBOSE)