I want to read regular expressions from a file, where each line contains a regex:
lorem.*
dolor\S*
The following code is supposed to read each and append it to a list of regex strings:
vocabulary=[]
with open(path, "r") as vocabularyFile:
for term in vocabularyFile:
term = term.rstrip()
vocabulary.append(term)
This code seems to escape the \ special character in the file as \\. How can I either avoid escaping or unescape the string so that it can be worked with as if I wrote this?
regex = r"dolor\S*"
You are getting confused by echoing the value. The Python interpreter echoes values by printing the repr() function result, and this makes sure to escape any meta characters:
>>> regex = r"dolor\S*"
>>> regex
'dolor\\S*'
regex is still an 8 character string, not 9, and the single character at index 5 is a single backslash:
>>> regex[4]
'r'
>>> regex[5]
'\\'
>>> regex[6]
'S'
Printing the string writes out all characters verbatim, so no escaping takes place:
>>> print(regex)
dolor\S*
The same process is applied to the contents of containers, like a list or a dict:
>>> container = [regex, 'foo\nbar']
>>> print(container)
['dolor\\S*', 'foo\nbar']
Note that I didn't echo there, I printed. str(list_object) produces the same output as repr(list_object) here.
If you were to print individual elements from the list, you get the same unescaped result again:
>>> print(container[0])
dolor\S*
>>> print(container[1])
foo
bar
Note how the \n in the second element was written out as a newline now. It is for that reason that containers use repr() for contents; to make otherwise hard-to-detect or non-printable data visible.
In other words, your strings do not contain escaped strings here.
Related
I'm encountering confusing and seemingly contradictory rules regarding raw strings. Consider the following example:
>>> text = 'm\n'
>>> match = re.search('m\n', text)
>>> print match.group()
m
>>> print text
m
This works, which is fine.
>>> text = 'm\n'
>>> match = re.search(r'm\n', text)
>>> print match.group()
m
>>> print text
m
Again, this works. But shouldn't this throw an error, because the raw string contains the characters m\n and the actual text contains a newline?
>>> text = r'm\n'
>>> match = re.search(r'm\n', text)
>>> print match.group()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'
>>> print text
m\n
The above, surprisingly, throws an error, even though both are raw strings. This means both contain just the text m\n with no newlines.
>>> text = r'm\n'
>>> match = re.search(r'm\\n', text)
>>> print text
m\n
>>> print match.group()
m\n
The above works, surprisingly. Why do I have to escape the backslash in the re.search, but not in the text itself?
Then there's backslash with normal characters that have no special behavior:
>>> text = 'm\&'
>>> match = re.search('m\&', text)
>>> print text
m\&
>>> print match.group()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'
This doesn't match, even though both the pattern and the string lack special characters.
In this situation, no combination of raw strings works (text as a raw string, patterns as a raw string, both or none).
However, consider the last example. Escaping in the text variable, 'm\\&', doesn't work, but escaping in the pattern does. This parallels the behavior above--even stranger, I feel, considering that \& is of no special meaning to either Python or re:
>>> text = 'm\&'
>>> match = re.search(r'm\\&', text)
>>> print text
m\&
>>> print match.group()
m\&
My understanding of raw strings is that they inhibit the behavior of the backslash in python. For regular expressions, this is important because it allows re.search to apply its own internal backslash behavior, and prevent conflicts with Python. However, in situations like the above, where backslash effectively means nothing, I'm not sure why it seems necessary. Worse yet, I don't understand why I need to backslash for the pattern, but not the text, and when I make both a raw string, it doesn't seem to work.
The docs don't provide much guidance in this regard. They focus on examples with obvious problems, such as '\section', where \s is a meta-character. Looking for a complete answer to prevent unanticipated behavior such as this.
In the regular Python string, 'm\n', the \n represents a single newline character, whereas in the raw string r'm\n' the \ and n are just themselves. So far, so simple.
If you pass the string 'm\n' as a pattern to re.search(), you're passing a two-character string (m followed by newline), and re will happily go and find instances of that two-character string for you.
If you pass the three-character string r'm\n', the re module itself will interpret the two characters \ n as having the special meaning "match a newline character", so that the whole pattern means "match an m followed by a newline", just as before.
In your third example, since the string r'm\n' doesn't contain a newline, there's no match:
>>> text = r'm\n'
>>> match = re.search(r'm\n', text)
>>> print(match)
None
With the pattern r'm\\n', you're passing two actual backslashes to re.search(), and again, the re module itself is interpreting the double backslash as "match a single backslash character".
In the case of 'm\&', something slightly different is going on. Python treats the backslash as a regular character, because it isn't part of an escape sequence. re, on the other hand, simply discards the \, so the pattern is effectively m&. You can see that this is true by testing the pattern against 'm&':
>>> re.search('m\&', 'm&').group()
'm&'
As before, doubling the backslash tells re to search for an actual backslash character:
>>> re.search(r'm\\&', 'm\&').group()
'm\\&'
... and just to make things a little more confusing, the single backslash is represented by Python doubled. You can see that it's actually a single backslash by printing it:
>>> print(re.search(r'm\\&', 'm\&').group())
m\&
To explain it in simple terms, \<character> has a special meaning in regular expressions. For example \s for whitespace characters, \d for decimal digits, \n for new-line characters, etc.
When you define a string as
s = 'foo\n'
This string contains the characters f, o, o and the new-line character (length 4).
However, when defining a raw string:
s = r'foo\n'
This string contains the characters f, o, o, \ and n (length 5).
When you compile a regexp with raw \n (i.e. r'\n'), it'll match all new lines. Similarly, just using the new-line character (i.e. '\n') it's going to match new-line characters just like a matches a and so on.
Once you understand this concept, you should be able to figure out the rest.
To elaborate a bit further. In order to match the back-slash character \ using regex, the valid regular expression is \\ which in Python would be r'\\' or its equivalent '\\\\'.
text = r'm\n'
match = re.search(r'm\\n', text)
First line using r stops python from interpreting \n as single byte.
Second line using r plays the same role as first.Using \ prevents regex from interpreting as \n .Regex also uses \ like \s, \d.
The following characters are the meta characters that give special meaning to the regular expression search syntax:
\ the backslash escape character.
The backslash gives special meaning to the character following it. For example, the combination "\n" stands for the newline, one of the control characters. The combination "\w" stands for a "word" character, one of the convenience escape sequences while "\1" is one of the substitution special characters.
Example: The regex "aa\n" tries to match two consecutive "a"s at the end of a line, inclusive the newline character itself.
Example: "a+" matches "a+" and not a series of one or "a"s.
In order to understand the internal representation of the strings you're confused about. I'd recommend you using repr and len builtin functions. Using those you'll be able to understand exactly how the strings are and you won't be confused anymore about pattern matching because you'll exactly know the internal representation. For instance, let's say you wanna analize the strings you're having troubles with:
use_cases = [
'm\n',
r'm\n',
'm\\n',
r'm\\n',
'm\&',
r'm\&',
'm\\&',
r'm\\&',
]
for u in use_cases:
print('-' * 10)
print(u, repr(u), len(u))
The output would be:
----------
m
'm\n' 2
----------
m\n 'm\\n' 3
----------
m\n 'm\\n' 3
----------
m\\n 'm\\\\n' 4
----------
m\& 'm\\&' 3
----------
m\& 'm\\&' 3
----------
m\& 'm\\&' 3
----------
m\\& 'm\\\\&' 4
So you can see exactly the differences between normal/raw strings.
s='\'(-inf-24.5]\'' #this in not working
what should be put before \ to include it?
we have to assign s '\'(-inf-24.5]\''
the last two characters are two single quotes and not a single double quote.
the string should literally contain the given single backslashes as the string is to be inserted as it is in a column.
You can try this:
>>> s="\\'(-inf-24.5]\\'"
>>> print s
\'(-inf-24.5]\'
or
>>> s="'\\'(-inf-24.5]\\''"
>>> print s
'\'(-inf-24.5]\''
Basically, you will need to escape the backslash, when you write \' normally, python treats it as the ' being escaped. Also, python strings can be either "", or '', so you can mix them togather to get the desired result.
>>> s = r"'\'(-inf-24.5]\''"
>>> s
"'\\'(-inf-24.5]\\''"
>>> print(s)
'\'(-inf-24.5]\''
Prepending r before a string denotes a raw string, basically indicating to the interpreter that that string's characters should be taken literally. The only thing it can't do is end a string with a backslash (such a backslash would have to be concatenated from a separate string).
I use a regex to build a list of all key-value pair present on line(string).
My key-pair syntax respect/match the following regex:
re.compile("\((.*?),(.*?)\)")
typically I have to parse a string like:
(hex, 0x123456)
If I use the interpreter it's OK
str = "(hex,0x123456)"
>>> KeyPair = re.findall(MyRegex, str)
>>> KeyPair
[('hex', '0x123456')]
But when I use that code under linux to parse a command line output I get:
[('hex', '0x123456\x00')]
it comes from the following code
KeyPayList = []
# some code ....
process = subprocess.Popen(self.cmd_line, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=False, stdin=subprocess.PIPE)
# here we parse the output
for line in process.stdout:
if line.startswith(lineStartWith):
KeyPair = re.findall(MyRegex, line.strip())
KeyPayList.append(KeyPair)
Do you know why I get that strange \x00 in the second group I captured ?
Note that I already try to strip the string before calling findall.
That's a null byte, and it is present in your original string. You may not have seen it, as your terminal will ignore it when you print the string:
>>> s = "(hex,0x123456\x00)"
>>> print s
(hex,0x123456)
The Python repr() function used for container contents (such as the contents of the tuple you are printing here) does show it:
>>> print repr(s)
'(hex,0x123456\x00)'
Your regular expression is simply returning that null byte because it is present in your original string:
>>> import re
>>> s = "(hex,0x123456\x00)"
>>> yourpattern = re.compile("\((.*?),(.*?)\)")
>>> yourpattern.search(s).groups()
('hex', '0x123456\x00')
If you were to remove it, the regular expression engine won't return it either:
>>> yourpattern.search(s.replace('\x00', '')).groups()
('hex', '0x123456')
It's simply that, in your case, the strings yielded by the process.stdout iterator contain null bytes.
Without a specific list of characters to remove, strip deletes whitespace characters. That means tab, linefeed, vertical tab, form feed, carriage return, and space.
Many of those aren't relevant to most applications, but if you want to remove null characters then you must say so explicitly. For instance, if you wanted to remove tabs, spaces, and nulls, then you would write
line.strip('\x00\x09\x20')
I'm trying to use pyparsing to parse quoted strings under the following conditions:
The quoted string might contain internal quotes.
I want to use backslashes to escape internal quotes.
The quoted string might end with a backslash.
I'm struggling to define a successful parser. Also, I'm starting to wonder whether the regular expression used by pyparsing for quoted strings of this kind is correct (see my alternative regular expression below).
Am I using pyparsing incorrectly (most likely) or is there a bug in pyparsing?
Here's a script that demonstrates the problem (Note: ignore this script; please focus instead on the Update below.):
import pyparsing as pp
import re
# A single-quoted string having:
# - Internal escaped quote.
# - A backslash as the last character before the final quote.
txt = r"'ab\'cd\'"
# Parse with pyparsing.
# Does not work as expected: grabs only first 3 characters.
parser = pp.QuotedString(quoteChar = "'", escChar = '\\', escQuote = '\\')
toks = parser.parseString(txt)
print
print 'txt: ', txt
print 'pattern:', parser.pattern
print 'toks: ', toks
# Parse with a regex just like the pyparsing pattern, but with
# the last two groups flipped -- which seems more correct to me.
# This works.
rgx = re.compile(r"\'(?:[^'\n\r\\]|(?:\\.)|(?:\\))*\'")
print
print rgx.search(txt).group(0)
Output:
txt: 'ab\'cd\'
pattern: \'(?:[^'\n\r\\]|(?:\\)|(?:\\.))*\'
toks: ["ab'"]
'ab\'cd\'
Update
Thanks for the replies. I suspect that I've confused things by framing my question badly, so let me try again.
Let's say we are trying to parse a language that uses quoting rules generally like Python's. We want users to be able to define strings that can include internal quotes (protected by backslashes) and we want those strings to be able to end with a backslash. Here's an example file in our language. Note that the file would also parse as valid Python syntax, and if we printed foo (in Python), the output would be the literal value: ab'cd\
# demo.txt
foo = 'ab\'cd\\'
My goal is to use pyparsing to parse such a language. Is there a way to do it? The question above is basically where I ended up after several failed attempts. Below is my initial attempt. It fails because there are two backslashes at the end, rather than just one.
with open('demo.txt') as fh:
txt = fh.read().split()[-1].strip()
parser = pp.QuotedString(quoteChar = "'", escChar = '\\')
toks = parser.parseString(txt)
print
print 'txt: ', txt
print 'pattern:', parser.pattern
print 'toks: ', toks # ["ab'cd\\\\"]
I guess the problem is that QuotedString treats the backslash only as a quote-escape whereas Python treats a backslash as a more general-purpose escape.
Is there a simple way to do this that I'm overlooking? One workaround that occurs to me is to use .setParseAction(...) to handle the double-backslashes after the fact -- perhaps like this, which seems to work:
qHandler = lambda s,l,t: [ t[0].replace('\\\\', '\\') ]
parser = pp.QuotedString(quoteChar = "'", escChar = '\\').setParseAction(qHandler)
I think you're misunderstanding the use of escQuote. According to the docs:
escQuote - special quote sequence to escape an embedded quote string (such as SQL's "" to escape an embedded ") (default=None)
So escQuote is for specifying a complete sequence that is parsed as a literal quote. In the example given in the docs, for instance, you would specify escQuote='""' and it would be parsed as ". By specifying a backslash as escQuote, you are causing a single backslash to be interpreted as a quotation mark. You don't see this in your example because you don't escape anything but quotes. However, if you try to escape something else, you'll see it won't work:
>>> txt = r"'a\Bc'"
>>> parser = pyp.QuotedString(quoteChar = "'", escChar = '\\', escQuote = "\\")
>>> parser.parseString(txt)
(["a'Bc"], {})
Notice that the backslash was replaced with '.
As for your alternative, I think the reason that pyparsing (and many other parsers) don't do this is that it involves special-casing one position within the string. In your regex, a single backslash is an escape character everywhere except as the last character in the string, in which position it is treated literally. This means that you cannot tell "locally" whether a given quote is really the end of the string or not --- even if it has a backslash, it might not be the end if there is one later on without a backslash. This can lead to parse ambiguities and surprising parsing behavior. For instance, consider these examples:
>>> txt = r"'ab\'xxxxxxx"
>>> print rgx.search(txt).group(0)
'ab\'
>>> txt = r"'ab\'xxxxxxx'"
>>> print rgx.search(txt).group(0)
'ab\'xxxxxxx'
By adding an apostrophe at the end of the string, I suddenly caused the earlier apostrophe to no longer be the end, and added all the xs to the string at once. In a real-usage context, this can lead to confusing situations in which mismatched quotes silently result in a reparsing of the string rather than a parse error.
Although I can't come up with an example at the moment, I also suspect that this has the possibility to cause "catastrophic backstracking" if you actually try to parse a sizable document containing multiple strings of this type. (This was my point about the "100MB of other text".) Because the parser can't know whether a given \' is the end of the string without parsing further, it might potentially have to go all the way to the end of the file just to make sure there are no more quote marks out there. If that remaining portion of the file contains additional strings of this type, it may become complicated to figure out which quotes are delimiting which strings. For instance, if the input contains something like
'one string \' 'or two'
we can't tell whether this is two valid strings (one string \ and or two) or one with invalid material after it (one string \' and the non-string tokens or two followed by an unmatched quote). This kind of situation is not desirable in many parsing contexts; you want the decisions about where strings begin and end to be locally determinable, and not depend on the occurrence of other tokens much later in the document.
What is it about this code that is not working for you?
from pyparsing import *
s = r"foo = 'ab\'cd\\'" # <--- IMPORTANT - use a raw string literal here
ident = Word(alphas)
strValue = QuotedString("'", escChar='\\')
strAssign = ident + '=' + strValue
results = strAssign.parseString(s)
print results.asList() # displays repr form of each element
for r in results:
print r # displays str form of each element
# count the backslashes
backslash = '\\'
print results[-1].count(backslash)
prints:
['foo', '=', "ab'cd\\\\"]
foo
=
ab'cd\\
2
EDIT:
So "\'" becomes just "'", but "\" is parsed but stays as "\" instead of being an escaped "\". Looks like a bug in QuotedString. For now you can add this workaround:
import re
strValue.setParseAction(lambda t: re.sub(r'\\(.)', r'\g<1>', t[0]))
Which will take every escaped character sequence and just give back the escaped character alone, without the leading '\'.
I'll add this in the next patch release of pyparsing.
PyParsing's QuotedString parser does not handle quoted strings that end with backslashes. This is a fundamental limitation, that doesn't have any easy workaround that I can see. If you want to support that kind of string, you'll need to use something other than QuotedString.
This is not an uncommon limitation either. Python itself does not allow an odd number of backslashes at the end of a "raw" string literal. Try it: r"foo\" will raise an exception, while r"bar\\" will include both backslashes in the output.
The reason you are getting truncated output (rather than an exception) from your current code is because you're passing a backslash as the escQuote parameter. I think that is intended to be an alternative to specifying an escape character, rather than a supplement. What is happening is that the first backslash is being interpreted as an internal quote (which it unescapes), and since it's followed by an actual quote character, the parser thinks it's reached the end of the quoted string. Thus you get ab' as your result.
This question already has answers here:
How can I put an actual backslash in a string literal (not use it for an escape sequence)?
(4 answers)
Closed 7 months ago.
In python, I am trying to replace a single backslash ("\") with a double backslash("\"). I have the following code:
directory = string.replace("C:\Users\Josh\Desktop\20130216", "\", "\\")
However, this gives an error message saying it doesn't like the double backslash. Can anyone help?
No need to use str.replace or string.replace here, just convert that string to a raw string:
>>> strs = r"C:\Users\Josh\Desktop\20130216"
^
|
notice the 'r'
Below is the repr version of the above string, that's why you're seeing \\ here.
But, in fact the actual string contains just '\' not \\.
>>> strs
'C:\\Users\\Josh\\Desktop\\20130216'
>>> s = r"f\o"
>>> s #repr representation
'f\\o'
>>> len(s) #length is 3, as there's only one `'\'`
3
But when you're going to print this string you'll not get '\\' in the output.
>>> print strs
C:\Users\Josh\Desktop\20130216
If you want the string to show '\\' during print then use str.replace:
>>> new_strs = strs.replace('\\','\\\\')
>>> print new_strs
C:\\Users\\Josh\\Desktop\\20130216
repr version will now show \\\\:
>>> new_strs
'C:\\\\Users\\\\Josh\\\\Desktop\\\\20130216'
Let me make it simple and clear. Lets use the re module in python to escape the special characters.
Python script :
import re
s = "C:\Users\Josh\Desktop"
print s
print re.escape(s)
Output :
C:\Users\Josh\Desktop
C:\\Users\\Josh\\Desktop
Explanation :
Now observe that re.escape function on escaping the special chars in the given string we able to add an other backslash before each backslash, and finally the output results in a double backslash, the desired output.
Hope this helps you.
Use escape characters: "full\\path\\here", "\\" and "\\\\"
In python \ (backslash) is used as an escape character. What this means that in places where you wish to insert a special character (such as newline), you would use the backslash and another character (\n for newline)
With your example string you would notice that when you put "C:\Users\Josh\Desktop\20130216" in the repl you will get "C:\\Users\\Josh\\Desktop\x8130216". This is because \2 has a special meaning in a python string. If you wish to specify \ then you need to put two \\ in your string.
"C:\\Users\\Josh\\Desktop\\28130216"
The other option is to notify python that your entire string must NOT use \ as an escape character by pre-pending the string with r
r"C:\Users\Josh\Desktop\20130216"
This is a "raw" string, and very useful in situations where you need to use lots of backslashes such as with regular expression strings.
In case you still wish to replace that single \ with \\ you would then use:
directory = string.replace(r"C:\Users\Josh\Desktop\20130216", "\\", "\\\\")
Notice that I am not using r' in the last two strings above. This is because, when you use the r' form of strings you cannot end that string with a single \
Why can't Python's raw string literals end with a single backslash?
https://pythonconquerstheuniverse.wordpress.com/2008/06/04/gotcha-%E2%80%94-backslashes-are-escape-characters/
Maybe a syntax error in your case,
you may change the line to:
directory = str(r"C:\Users\Josh\Desktop\20130216").replace('\\','\\\\')
which give you the right following output:
C:\\Users\\Josh\\Desktop\\20130216
The backslash indicates a special escape character. Therefore, directory = path_to_directory.replace("\", "\\") would cause Python to think that the first argument to replace didn't end until the starting quotation of the second argument since it understood the ending quotation as an escape character.
directory=path_to_directory.replace("\\","\\\\")
Given the source string, manipulation with os.path might make more sense, but here's a string solution;
>>> s=r"C:\Users\Josh\Desktop\\20130216"
>>> '\\\\'.join(filter(bool, s.split('\\')))
'C:\\\\Users\\\\Josh\\\\Desktop\\\\20130216'
Note that split treats the \\ in the source string as a delimited empty string. Using filter gets rid of those empty strings so join won't double the already doubled backslashes. Unfortunately, if you have 3 or more, they get reduced to doubled backslashes, but I don't think that hurts you in a windows path expression.
You could use
os.path.abspath(path_with_backlash)
it returns the path with \
Use:
string.replace(r"C:\Users\Josh\Desktop\20130216", "\\", "\\")
Escape the \ character.