這學期在學習編譯原理，最近的上機作業就是做一個簡單的詞法分析器，在做的過程中，突然有個需求就是判斷一個字串是否為合法的標示符，因為我是用python語言做的，做的是Python的詞法分析器，於是下面分享以下怎樣判斷一個字串是合法的標示符。

首先，我們來熟悉以下python標示符的定義是什麼？

定義：以字母或下劃線開始的，由字母，數字或下劃線組成，但是不能是python的保留字。

又有疑問了，python有哪些保留字，分別是什麼？

# python2.x
import keyword
print keyword.kwlis
# python3.x
import keyword
print(keyword.kwlist)


# python2.x輸出：
 ['and','as','assert','break','class','continue','def','del','elif','else','except','exec','finally','for','from','global','if','import','in','is','lambda','not','or','pass','print','raise','return','try','while','with','yield']
# 共31個
# python3.x輸出：
['False','None','True','and','nonlocal','yield']
# 共33個

好了，下面開始判斷

# python2.7

#!/usr/bin/env python
# -*- coding: UTF-8 -*-
import keyword
import string
def is_signal(s):
 kw = keyword.kwlist
 if s in kw:
  return 0
 elif s[0] == '_' or s[0] in string.letters: # 判斷是否為字母或下劃線開頭
  for i in s:
   if i == '_' or i in string.letters or i in string.digits: # 判斷是否由字母數字或下劃線組成
    pass
   else:
    return 0
  return 1
 else:
  return 0
def main():
 s = raw_input()
 if is_signal(s) == 1:
  print "True"
 else:
  print "False"
if __name__ == '__main__':
 main()

# python3.4

#!/usr/bin/env python
# -*- coding: UTF-8 -*-
import keyword
import string
def is_signal(s):
 kw = keyword.kwlist
 if s in kw:
  return 0
 elif s[0] == '_' or s[0] in string.ascii_letters: # 判斷是否為字母或下劃線開頭
  for i in s:
   if i == '_' or i in string.ascii_letters or i in string.digits: # 判斷是否由字母數字或下劃線組成
    pass
   else:
    return 0
  return 1
 else:
  return 0
def main():
 s = input()
 if is_signal(s) == 1:
  print("True")
 else:
  print("False")
if __name__ == '__main__':
 main()

通過鍵盤輸入判斷，是標示符，則返回True，否則返回False

補充知識：python:識別符號必須以字母或下劃線開頭，後面跟字母,下劃線或者數字

標識符合法性檢查，首先要以字母或者下劃線開始，後面要跟字母,下劃線或者或數字.這個小例子只檢查長度大於等於 2 的識別符號

idcheck.py

#!/usr/bin/env python
'''
idcheck.py -- checks identifiers for validity
'''
 
import string    # string utility module
 
# create alphabet and number sets
alphas = string.ascii_letters + '_'
nums = string.digits
 
# salutation message and input prompt
print ('Welcome to the Identifier Checker v1.0')
print ('Testees must be at least 2 chars long.')
inp = input('Identifier to test ?')
 
if len(inp) >= 1:
 
  if inp[0] not in alphas:
    print ('invalid: first symbol must be alphabetic')
 
  else:
    for otherChar in inp[1:]:
      if otherChar not in alphas + nums:
        print ('invalid: remaining symbols must be alphanumeric')
        break
    else:
      print ("okay as an identifier")
else:
  print ('invalid: length must be >= 1')

執行結果 1：

Welcome to the Identifier Checker v1.0
Testees must be at least 2 chars long.
Identifier to test -> 123_das
invalid: first symbol must be alphabetic

執行結果 2：

Welcome to the Identifier Checker v1.0
Testees must be at least 2 chars long.
Identifier to test -> _123sdad
okay as an identifier

以上這篇Python判斷字串是否為合法標示符操作就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支援我們。