在許多的情況下，很多要匹配內容是一起出現，或者一起不出現的。比如《》，< >，這樣的括號，不存在使用半個的情況。因此，在正則表示式裡也有一致性的判斷，要麼兩個尖括號一起出現，要麼一個也不要出現。怎麼樣來實現這種判斷呢？針對這種情況得引入新的正則表示式的語法：(?=pattern)，這個語法它會向前搜尋或者向後搜尋相關內容，如果不會出現就不能匹配。不過，這個匹配不會消耗任何輸入的字元，它只是檢視一下。例子如下：
 

#python 3.6
#蔡軍生 
#http://blog.csdn.net/caimouse/article/details/51749579
#
import re

address = re.compile(
    '''
    # A name is made up of letters, and may include "."
    # for title abbreviations and middle initials.
    ((?P<name>
       ([\w.,]+\s+)*[\w.,]+
     )
     \s+
    ) # name is no longer optional

    # LOOKAHEAD
    # Email addresses are wrapped in angle brackets, but only
    # if both are present or neither is.
    (?= (<.*>$)       # remainder wrapped in angle brackets
        |
        ([^<].*[^>]$) # remainder *not* wrapped in angle brackets
      )

    <? # optional opening angle bracket

    # The address itself: 
 
[email protected]
    (?P<email>
      [\w\d.+-]+       # username
      @
      ([\w\d.]+\.)+    # domain name prefix
      (com|org|edu)    # limit the allowed top-level domains
    )

    >? # optional closing angle bracket
    ''',
    re.VERBOSE)

candidates = [
    u'First Last <
 
[email protected]>',
    u'No Brackets [email protected]',
    u'Open Bracket <[email protected]',
    u'Close Bracket [email protected]>',
]

for candidate in candidates:
    print('Candidate:', candidate)
    match = address.search(candidate)
    if match:
        print('  Name :', match.groupdict()['name'])
        print('  Email:', match.groupdict()['email'])
    else:
        print('  No match')
 

結果輸出如下：
Candidate: First Last <[email protected]>
  Name : First Last
  Email: [email protected]
Candidate: No Brackets [email protected]
  Name : No Brackets
  Email: [email protected]
Candidate: Open Bracket <[email protected]
  No match
Candidate: Close Bracket [email protected]>

  No match

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