Regular Expressions in Python

[fossil_blue]

Dear Gurus,

I am trying to find out how to write an effective regular expression
in python for the following scenario:

"any number of leading spaces at the beginning of a line" "follow
by a string" "there maybe a string that starts with *"

for example:

END *This is a comment

but I don't want to match this:

END e * This is a line with an error (e)

thanks,
Noel

 

[Jeff Epler]

opt_spaces = " *"
identifier = "[A-Za-z_][A-Za-z0-9_]+"
comment = "\*.*"
opt_comment = "(%s)?" % comment

pat = re.compile(opt_spaces + identifier + opt_spaces + opt_comment + "$")

for test in (
" END *This is a comment",
" END e * This is a line with an error (e)"):
print test, pat.match(test)

Jeff

 

[Paul McGuire]

opt_spaces = " *"
identifier = "[A-Za-z_][A-Za-z0-9_]+"
comment = "\*.*"
opt_comment = "(%s)?" % comment

pat = re.compile(opt_spaces + identifier + opt_spaces + opt_comment + "$")

for test in (
" END *This is a comment",
" END e * This is a line with an error (e)"):
print test, pat.match(test)

Jeff

Assuming you're more interested in the identifier than in the comment,
change identifier to "([A-Za-z_][A-Za-z0-9_]+)" so that the keyword gets
saved in the pat.match.groups() list.

-- Paul

 

[Paul McGuire]

Dear Gurus,

I am trying to find out how to write an effective regular expression
in python for the following scenario:

"any number of leading spaces at the beginning of a line" "follow
by a string" "there maybe a string that starts with *"

for example:

END *This is a comment

but I don't want to match this:

END e * This is a line with an error (e)

thanks,
Noel

Here's an example with sample code using both re's and pyparsing. Note that
the single .ignore() call takes care of ignoring comments on all contained
grammar constructs, and non-significant whitespace is implicitly ignored (so
no need to litter your matching expressions with lots of opt_spaces-type
content).

-- Paul
========================
from pyparsing import Word, alphas, alphanums, restOfLine, LineEnd,
ParseException

testdata = """
END *This is a comment
END*This is a comment (but the next line has no comment)
END
END e * This is a line with an error (e)"""
enquote = lambda st : ( '"%s"' % st )

print "test with pyparsing"
grammar = Word( alphas, alphanums ).setName("keyword") + LineEnd()
comment = "*" + restOfLine
grammar.ignore( comment )

for test in testdata.split("\n"):
try:
print enquote(test),"\n->",
print grammar.parseString( test )
except ParseException, pe:
print pe

print

import re
print "test with re"
opt_spaces = " *"
#identifier = "[A-Za-z_][A-Za-z0-9_]+" - I'm guessing this regexp should
have ()'s for accessing content as a group
identifier = "([A-Za-z_][A-Za-z0-9_]+)"
comment = "\*.*"
opt_comment = "(%s)?" % comment

pat = re.compile(opt_spaces + identifier + opt_spaces + opt_comment + "$")

for test in testdata.split("\n"):
print enquote(test),"\n->",
if pat.match(test):
print pat.match(test).groups()
else:
print "Bad text"

========================
Gives this output:

test with pyparsing
""
-> Expected keyword (0), (1,1)
"END *This is a comment"
-> ['END']
" END*This is a comment (but the next line has no comment)"
-> ['END']
" END"
-> ['END']
" END e * This is a line with an error (e)"
-> Expected end of line (8), (1,9)

test with re
""
-> Bad text
"END *This is a comment"
-> ('END', '*This is a comment')
" END*This is a comment (but the next line has no comment)"
-> ('END', '*This is a comment (but the next line has no comment)')
" END"
-> ('END', None)
" END e * This is a line with an error (e)"
-> Bad text