This semester in the study of the principles of compilation, the recent on-line homework is to do a simple lexical analyzer, in the process of doing, suddenly there is a need to determine whether a string is a legal marker, because I am using python language to do the Python lexical analyzer, and so the following share the following how to determine a string is a legal marker.
First, let's familiarize ourselves with what the following python markers are defined as.
Definition: starts with a letter or underscore, consists of letters, numbers, or underscores, but cannot be a python reserved word.
Another question, what are the reserved words in python and what are they?
# import keyword print # import keyword print() # Output: ['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'] # 31 in total # Output: ['False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield'] # 33 in total
Okay, here's the judgment call.
# python2.7
#!/usr/bin/env python
# -*- coding: UTF-8 -*-
import keyword
import string
def is_signal(s):
kw =
if s in kw:
return 0
elif s[0] == '_' or s[0] in : # Determine if it starts with a letter or an underscore
for i in s:
if i == '_' or i in or i in : # Determine if it consists of alphanumerics or underscores
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 =
if s in kw:
return 0
elif s[0] == '_' or s[0] in string.ascii_letters: # Determine if it starts with a letter or an underscore
for i in s:
if i == '_' or i in string.ascii_letters or i in : # Determine if it consists of alphanumerics or underscores
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()
Judge by the keyboard input, is a marker, then return True, otherwise return False
Additional knowledge:python:Identifiers must start with a letter or an underscore, followed by a letter, underscore, or number.
Identifiers are checked for legality by starting with a letter or underscore followed by a letter, underscore or number. This small example only checks identifiers of length greater than or equal to 2.
#!/usr/bin/env python
'''
-- checks identifiers for validity
'''
import string # string utility module
# create alphabet and number sets
alphas = string.ascii_letters + '_'
nums =
# 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')
Run Results 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
Run Results 2:
Welcome to the Identifier Checker v1.0 Testees must be at least 2 chars long. Identifier to test -> _123sdad okay as an identifier
Above this Python to determine whether the string is a legal marker operation is all that I have shared with you, I hope to be able to give you a reference, and I hope you will support me more.