Related
Technically, any odd number of backslashes, as described in the documentation.
>>> r'\'
File "<stdin>", line 1
r'\'
^
SyntaxError: EOL while scanning string literal
>>> r'\\'
'\\\\'
>>> r'\\\'
File "<stdin>", line 1
r'\\\'
^
SyntaxError: EOL while scanning string literal
It seems like the parser could just treat backslashes in raw strings as regular characters (isn't that what raw strings are all about?), but I'm probably missing something obvious.
The whole misconception about python's raw strings is that most of people think that backslash (within a raw string) is just a regular character as all others. It is NOT. The key to understand is this python's tutorial sequence:
When an 'r' or 'R' prefix is present, a character following a
backslash is included in the string without change, and all
backslashes are left in the string
So any character following a backslash is part of raw string. Once parser enters a raw string (non Unicode one) and encounters a backslash it knows there are 2 characters (a backslash and a char following it).
This way:
r'abc\d' comprises a, b, c, \, d
r'abc\'d' comprises a, b, c, \, ', d
r'abc\'' comprises a, b, c, \, '
and:
r'abc\' comprises a, b, c, \, ' but there is no terminating quote now.
Last case shows that according to documentation now a parser cannot find closing quote as the last quote you see above is part of the string i.e. backslash cannot be last here as it will 'devour' string closing char.
The reason is explained in the part of that section which I highlighted in bold:
String quotes can be escaped with a
backslash, but the backslash remains
in the string; for example, r"\"" is a
valid string literal consisting of two
characters: a backslash and a double
quote; r"\" is not a valid string
literal (even a raw string cannot end
in an odd number of backslashes).
Specifically, a raw string cannot end
in a single backslash (since the
backslash would escape the following
quote character). Note also that a
single backslash followed by a newline
is interpreted as those two characters
as part of the string, not as a line
continuation.
So raw strings are not 100% raw, there is still some rudimentary backslash-processing.
That's the way it is! I see it as one of those small defects in python!
I don't think there's a good reason for it, but it's definitely not parsing; it's really easy to parse raw strings with \ as a last character.
The catch is, if you allow \ to be the last character in a raw string then you won't be able to put " inside a raw string. It seems python went with allowing " instead of allowing \ as the last character.
However, this shouldn't cause any trouble.
If you're worried about not being able to easily write windows folder pathes such as c:\mypath\ then worry not, for, you can represent them as r"C:\mypath", and, if you need to append a subdirectory name, don't do it with string concatenation, for it's not the right way to do it anyway! use os.path.join
>>> import os
>>> os.path.join(r"C:\mypath", "subfolder")
'C:\\mypath\\subfolder'
In order for you to end a raw string with a slash I suggest you can use this trick:
>>> print r"c:\test"'\\'
test\
It uses the implicit concatenation of string literals in Python and concatenates one string delimited with double quotes with another that is delimited by single quotes. Ugly, but works.
Another trick is to use chr(92) as it evaluates to "\".
I recently had to clean a string of backslashes and the following did the trick:
CleanString = DirtyString.replace(chr(92),'')
I realize that this does not take care of the "why" but the thread attracts many people looking for a solution to an immediate problem.
Since \" is allowed inside the raw string. Then it can't be used to identify the end of the string literal.
Why not stop parsing the string literal when you encounter the first "?
If that was the case, then \" wouldn't be allowed inside the string literal. But it is.
The reason for why r'\' is syntactical incorrect is that although the string expression is raw the used quotes (single or double) always have to be escape since they would mark the end of the quote otherwise. So if you want to express a single quote inside single quoted string, there is no other way than using \'. Same applies for double quotes.
But you could use:
'\\'
Another user who has since deleted their answer (not sure if they'd like to be credited) suggested that the Python language designers may be able to simplify the parser design by using the same parsing rules and expanding escaped characters to raw form as an afterthought (if the literal was marked as raw).
I thought it was an interesting idea and am including it as community wiki for posterity.
Naive raw strings
The naive idea of a raw string is
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
and it will mean itself.
Unfortunately, this does not work, because if the whatever
happens to contain a quote, the raw string would end at that point.
It is simply impossible that I can put "whatever I want"
between fixed delimiters, because some of it could look like
the terminating delimiter -- no matter what that delimiter is.
Real-world raw strings (variant 1)
One possible approach to this problem would be to say
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
This restriction sounds harsh, until one recognizes that
Python's large offering of quotes can accommodate most situations
with this rule. The following are all valid Python quotes:
'
"
'''
"""
With this many possibilities for the delimiter, almost anything
can be made to work.
About the only exception would be if the string
literal is supposed to contain a complete list of all allowed
Python quotes.
Real-world raw strings (variant 2, as in Python)
Python, however, takes a different route using
an extended version of the above rule.
It effectively states
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
If I insist on including a quote, even that is allowed,
but I have to put a backslash before it.
So the Python approach is, in a sense, even more liberal
than variant 1 above -- but it has the side effect of
"mis"interpreting the closing quote as part of the string
if the last intended character of the string is a backslash.
Variant 2 is not helpful:
If I want the quote in my string,
but not the backslash, the allowed version of my string literal
will not be what I need.
However, given the three different other kinds of quotes I have
at my disposal, I will probably just pick one of those and my
problem will be solved -- so this is not problematic case.
The problematic case is this one:
If I want my string to end with a backslash, I am at a loss.
I need to resort to concatenating a non-raw string literal
containing the backslash.
Conclusion
After writing this, I go with several of the other posters
that variant 1 would have been easier to understand and to accept
and therefore more pythonic. That's life!
Comming from C it pretty clear to me that a single \ works as escape character allowing you to put special characters such as newlines, tabs and quotes into strings.
That does indeed disallow \ as last character since it will escape the " and make the parser choke. But as pointed out earlier \ is legal.
some tips :
1) if you need to manipulate backslash for path then standard python module os.path is your friend. for example :
os.path.normpath('c:/folder1/')
2) if you want to build strings with backslash in it BUT without backslash at the END of your string then raw string is your friend (use 'r' prefix before your literal string). for example :
r'\one \two \three'
3) if you need to prefix a string in a variable X with a backslash then you can do this :
X='dummy'
bs=r'\ ' # don't forget the space after backslash or you will get EOL error
X2=bs[0]+X # X2 now contains \dummy
4) if you need to create a string with a backslash at the end then combine tip 2 and 3 :
voice_name='upper'
lilypond_display=r'\DisplayLilyMusic \ ' # don't forget the space at the end
lilypond_statement=lilypond_display[:-1]+voice_name
now lilypond_statement contains "\DisplayLilyMusic \upper"
long live python ! :)
n3on
Despite its role, even a raw string cannot end in a single
backslash, because the backslash escapes the following quote
character—you still must escape the surrounding quote character to
embed it in the string. That is, r"...\" is not a valid string
literal—a raw string cannot end in an odd number of backslashes.
If you need to end a raw string with a single backslash, you can use
two and slice off the second.
I encountered this problem and found a partial solution which is good for some cases. Despite python not being able to end a string with a single backslash, it can be serialized and saved in a text file with a single backslash at the end. Therefore if what you need is saving a text with a single backslash on you computer, it is possible:
x = 'a string\\'
x
'a string\\'
# Now save it in a text file and it will appear with a single backslash:
with open("my_file.txt", 'w') as h:
h.write(x)
BTW it is not working with json if you dump it using python's json library.
Finally, I work with Spyder, and I noticed that if I open the variable in spider's text editor by double clicking on its name in the variable explorer, it is presented with a single backslash and can be copied to the clipboard that way (it's not very helpful for most needs but maybe for some..).
Technically, any odd number of backslashes, as described in the documentation.
>>> r'\'
File "<stdin>", line 1
r'\'
^
SyntaxError: EOL while scanning string literal
>>> r'\\'
'\\\\'
>>> r'\\\'
File "<stdin>", line 1
r'\\\'
^
SyntaxError: EOL while scanning string literal
It seems like the parser could just treat backslashes in raw strings as regular characters (isn't that what raw strings are all about?), but I'm probably missing something obvious.
The whole misconception about python's raw strings is that most of people think that backslash (within a raw string) is just a regular character as all others. It is NOT. The key to understand is this python's tutorial sequence:
When an 'r' or 'R' prefix is present, a character following a
backslash is included in the string without change, and all
backslashes are left in the string
So any character following a backslash is part of raw string. Once parser enters a raw string (non Unicode one) and encounters a backslash it knows there are 2 characters (a backslash and a char following it).
This way:
r'abc\d' comprises a, b, c, \, d
r'abc\'d' comprises a, b, c, \, ', d
r'abc\'' comprises a, b, c, \, '
and:
r'abc\' comprises a, b, c, \, ' but there is no terminating quote now.
Last case shows that according to documentation now a parser cannot find closing quote as the last quote you see above is part of the string i.e. backslash cannot be last here as it will 'devour' string closing char.
The reason is explained in the part of that section which I highlighted in bold:
String quotes can be escaped with a
backslash, but the backslash remains
in the string; for example, r"\"" is a
valid string literal consisting of two
characters: a backslash and a double
quote; r"\" is not a valid string
literal (even a raw string cannot end
in an odd number of backslashes).
Specifically, a raw string cannot end
in a single backslash (since the
backslash would escape the following
quote character). Note also that a
single backslash followed by a newline
is interpreted as those two characters
as part of the string, not as a line
continuation.
So raw strings are not 100% raw, there is still some rudimentary backslash-processing.
That's the way it is! I see it as one of those small defects in python!
I don't think there's a good reason for it, but it's definitely not parsing; it's really easy to parse raw strings with \ as a last character.
The catch is, if you allow \ to be the last character in a raw string then you won't be able to put " inside a raw string. It seems python went with allowing " instead of allowing \ as the last character.
However, this shouldn't cause any trouble.
If you're worried about not being able to easily write windows folder pathes such as c:\mypath\ then worry not, for, you can represent them as r"C:\mypath", and, if you need to append a subdirectory name, don't do it with string concatenation, for it's not the right way to do it anyway! use os.path.join
>>> import os
>>> os.path.join(r"C:\mypath", "subfolder")
'C:\\mypath\\subfolder'
In order for you to end a raw string with a slash I suggest you can use this trick:
>>> print r"c:\test"'\\'
test\
It uses the implicit concatenation of string literals in Python and concatenates one string delimited with double quotes with another that is delimited by single quotes. Ugly, but works.
Another trick is to use chr(92) as it evaluates to "\".
I recently had to clean a string of backslashes and the following did the trick:
CleanString = DirtyString.replace(chr(92),'')
I realize that this does not take care of the "why" but the thread attracts many people looking for a solution to an immediate problem.
Since \" is allowed inside the raw string. Then it can't be used to identify the end of the string literal.
Why not stop parsing the string literal when you encounter the first "?
If that was the case, then \" wouldn't be allowed inside the string literal. But it is.
The reason for why r'\' is syntactical incorrect is that although the string expression is raw the used quotes (single or double) always have to be escape since they would mark the end of the quote otherwise. So if you want to express a single quote inside single quoted string, there is no other way than using \'. Same applies for double quotes.
But you could use:
'\\'
Another user who has since deleted their answer (not sure if they'd like to be credited) suggested that the Python language designers may be able to simplify the parser design by using the same parsing rules and expanding escaped characters to raw form as an afterthought (if the literal was marked as raw).
I thought it was an interesting idea and am including it as community wiki for posterity.
Naive raw strings
The naive idea of a raw string is
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
and it will mean itself.
Unfortunately, this does not work, because if the whatever
happens to contain a quote, the raw string would end at that point.
It is simply impossible that I can put "whatever I want"
between fixed delimiters, because some of it could look like
the terminating delimiter -- no matter what that delimiter is.
Real-world raw strings (variant 1)
One possible approach to this problem would be to say
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
This restriction sounds harsh, until one recognizes that
Python's large offering of quotes can accommodate most situations
with this rule. The following are all valid Python quotes:
'
"
'''
"""
With this many possibilities for the delimiter, almost anything
can be made to work.
About the only exception would be if the string
literal is supposed to contain a complete list of all allowed
Python quotes.
Real-world raw strings (variant 2, as in Python)
Python, however, takes a different route using
an extended version of the above rule.
It effectively states
If I put an r in front of a pair of quotes,
I can put whatever I want between the quotes
as long as it does not contain a quote
and it will mean itself.
If I insist on including a quote, even that is allowed,
but I have to put a backslash before it.
So the Python approach is, in a sense, even more liberal
than variant 1 above -- but it has the side effect of
"mis"interpreting the closing quote as part of the string
if the last intended character of the string is a backslash.
Variant 2 is not helpful:
If I want the quote in my string,
but not the backslash, the allowed version of my string literal
will not be what I need.
However, given the three different other kinds of quotes I have
at my disposal, I will probably just pick one of those and my
problem will be solved -- so this is not problematic case.
The problematic case is this one:
If I want my string to end with a backslash, I am at a loss.
I need to resort to concatenating a non-raw string literal
containing the backslash.
Conclusion
After writing this, I go with several of the other posters
that variant 1 would have been easier to understand and to accept
and therefore more pythonic. That's life!
Comming from C it pretty clear to me that a single \ works as escape character allowing you to put special characters such as newlines, tabs and quotes into strings.
That does indeed disallow \ as last character since it will escape the " and make the parser choke. But as pointed out earlier \ is legal.
some tips :
1) if you need to manipulate backslash for path then standard python module os.path is your friend. for example :
os.path.normpath('c:/folder1/')
2) if you want to build strings with backslash in it BUT without backslash at the END of your string then raw string is your friend (use 'r' prefix before your literal string). for example :
r'\one \two \three'
3) if you need to prefix a string in a variable X with a backslash then you can do this :
X='dummy'
bs=r'\ ' # don't forget the space after backslash or you will get EOL error
X2=bs[0]+X # X2 now contains \dummy
4) if you need to create a string with a backslash at the end then combine tip 2 and 3 :
voice_name='upper'
lilypond_display=r'\DisplayLilyMusic \ ' # don't forget the space at the end
lilypond_statement=lilypond_display[:-1]+voice_name
now lilypond_statement contains "\DisplayLilyMusic \upper"
long live python ! :)
n3on
Despite its role, even a raw string cannot end in a single
backslash, because the backslash escapes the following quote
character—you still must escape the surrounding quote character to
embed it in the string. That is, r"...\" is not a valid string
literal—a raw string cannot end in an odd number of backslashes.
If you need to end a raw string with a single backslash, you can use
two and slice off the second.
I encountered this problem and found a partial solution which is good for some cases. Despite python not being able to end a string with a single backslash, it can be serialized and saved in a text file with a single backslash at the end. Therefore if what you need is saving a text with a single backslash on you computer, it is possible:
x = 'a string\\'
x
'a string\\'
# Now save it in a text file and it will appear with a single backslash:
with open("my_file.txt", 'w') as h:
h.write(x)
BTW it is not working with json if you dump it using python's json library.
Finally, I work with Spyder, and I noticed that if I open the variable in spider's text editor by double clicking on its name in the variable explorer, it is presented with a single backslash and can be copied to the clipboard that way (it's not very helpful for most needs but maybe for some..).
Could you tell me why '?\\\?'=='?\\\\?' gives True? That drives me crazy and I can't find a reasonable answer...
>>> list('?\\\?')
['?', '\\', '\\', '?']
>>> list('?\\\\?')
['?', '\\', '\\', '?']
Basically, because python is slightly lenient in backslash processing. Quoting from https://docs.python.org/2.0/ref/strings.html :
Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, i.e., the backslash is left in the string.
(Emphasis in the original)
Therefore, in python, it isn't that three backslashes are equal to four, it's that when you follow backslash with a character like ?, the two together come through as two characters, because \? is not a recognized escape sequence.
This is because backslash acts as an escape character for the character(s) immediately following it, if the combination represents a valid escape sequence. The dozen or so escape sequences are listed here. They include the obvious ones such as newline \n, horizontal tab \t, carriage return \r and more obscure ones such as named unicode characters using \N{...}, e.g. \N{WAVY DASH} which represents unicode character \u3030. The key point though is that if the escape sequence is not known, the character sequence is left in the string as is.
Part of the problem might also be that the Python interpreter output is misleading you. This is because the backslashes are escaped when displayed. However, if you print those strings, you will see the extra backslashes disappear.
>>> '?\\\?'
'?\\\\?'
>>> print('?\\\?')
?\\?
>>> '?\\\?' == '?\\?' # I don't know why you think this is True???
False
>>> '?\\\?' == r'?\\?' # but if you use a raw string for '?\\?'
True
>>> '?\\\\?' == '?\\\?' # this is the same string... see below
True
For your specific examples, in the first case '?\\\?', the first \ escapes the second backslash leaving a single backslash, but the third backslash remains as a backslash because \? is not a valid escape sequence. Hence the resulting string is ?\\?.
For the second case '?\\\\?', the first backslash escapes the second, and the third backslash escapes the fourth which results in the string ?\\?.
So that's why three backslashes is the same as four:
>>> '?\\\?' == '?\\\\?'
True
If you want to create a string with 3 backslashes you can escape each backslash:
>>> '?\\\\\\?'
'?\\\\\\?'
>>> print('?\\\\\\?')
?\\\?
or you might find "raw" strings more understandable:
>>> r'?\\\?'
'?\\\\\\?'
>>> print(r'?\\\?')
?\\\?
This turns of escape sequence processing for the string literal. See String Literals for more details.
Because \x in a character string, when x is not one of the special backslashable characters like n, r, t, 0, etc, evaluates to a string with a backslash and then an x.
>>> '\?'
'\\?'
From the python lexical analysis page under string literals at:
https://docs.python.org/2/reference/lexical_analysis.html
There is a table that lists all the recognized escape sequences.
\\ is an escape sequence that is === \
\? is not an escape sequence and is === \?
so '\\\\' is '\\' followed by '\\' which is '\\' (two escaped \)
and '\\\' is '\\' followed by '\' which is also '\\' (one escaped \ and one raw \)
also, it should be noted that python does not distinguish between single and double quotes surrounding a string literal, unlike some other languages.
So 'String' and "String" are the exact same thing in python, they do not affect the interpretation of escape sequences.
mhawke's answer pretty much covers it, I just want to restate it in a more concise form and with minimal examples that illustrate this behaviour.
I guess one thing to add is that escape processing moves from left to right, so that \n first finds the backslash and then looks for a character to escape, then finds n and escapes it; \\n finds first backslash, finds second and escapes it, then finds n and sees it as a literal n; \? finds backslash and looks for a char to escape, finds ? which cannot be escaped, and so treats \ as a literal backslash.
As mhawke noted, the key here is that interactive interpreter escapes the backslash when displaying a string. I'm guessing the reason for that is to ensure that text strings copied from interpreter into code editor are valid python strings. However, in this case this allowance for convenience causes confusion.
>>> print('\?') # \? is not a valid escape code so backslash is left as-is
\?
>>> print('\\?') # \\ is a valid escape code, resulting in a single backslash
'\?'
>>> '\?' # same as first example except that interactive interpreter escapes the backslash
\\?
>>> '\\?' # same as second example, backslash is again escaped
\\?
i am very new to regular expression and trying get "\" character using python
normally i can escape "\" like this
print ("\\");
print ("i am \\nit");
output
\
i am \nit
but when i use the same in regX it didn't work as i thought
print (re.findall(r'\\',"i am \\nit"));
and return me output
['\\']
can someone please explain why
EDIT: The problem is actually how print works with lists & strings. It prints the representation of the string, not the string itself, the representation of a string containing just a backslash is '\\'. So findall is actually finding the single backslash correctly, but print isn't printing it as you'd expect. Try:
>>> print(re.findall(r'\\',"i am \\nit")[0])
\
(The following is my original answer, it can be ignored (it's entirely irrelevant), I'd misinterpreted the question initially. But it seems to have been upvoted a bit, so I'll leave it here.)
The r prefix on a string means the string is in "raw" mode, that is, \ are not treated as special characters (it doesn't have anything to do with "regex").
However, r'\' doesn't work, as you can't end a raw string with a backslash, it's stated in the docs:
Even in a raw string, string quotes can be escaped with a backslash, but the backslash remains in the string; for example, r"\"" is a valid string literal consisting of two characters: a backslash and a double quote; r"\" is not a valid string literal (even a raw string cannot end in an odd number of backslashes). Specifically, a raw string cannot end in a single backslash (since the backslash would escape the following quote character).
But you actually can use a non-raw string to get a single backslash: "\\".
can someone please explain why
Because re.findall found one match, and the match text consisted of a backslash. It gave you a list with one element, which is a string, which has one character, which is a backslash.
That is written ['\\'] because '\\' is how you write "a string with one backslash" - just like you had to do when you wrote the example code print "\\".
Note that you're using two different kinds of string literal here -- there's the regular string "a string" and the raw string r"a raw string". Regular string literals observe backslash escaping, so to actually put a backslash in the string, you need to escape it too. Raw string literals treat backslashes like any other character, so you're more limited in which characters you can actually put in the string (no specials that need an escape code) but it's easier to enter things like regular expressions, because you don't need to double up backslashes if you need to add a backslash to have meaning inside the string, not just when creating the string.
It is unnecessary to escape backslashes in raw strings, unless the backslash immediately precedes the closing quote.
I am trying to match (using regex in python):
http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg
in the following string:
http://www.mymaterialssite.com','http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg','Model Photo'
My code has something like this:
temp="http://www.mymaterialssite.com','http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg','Model Photo'"
dummy=str(re.compile(r'.com'',,''(.*?)'',,''Model Photo').search(str(temp)).group(1))
I do not think the "dummy" is correct & I am unsure how I "escape" the single and double quotes in the regex re.compile command.
I tried googling for the problem, but I couldnt find anything relevant.
Would appreciate any guidance on this.
Thanks.
The easiest way to deal with strings in Python that contain escape characters and quotes is to triple double-quote the string (""") and prefix it with r. For example:
my_str = r"""This string would "really "suck"" to write if I didn't
know how to tell Python to parse it as "raw" text with the 'r' character and
triple " quotes. Especially since I want \n to show up as a backlash followed
by n. I don't want \0 to be the null byte either!"""
The r means "take escape characters as literal". The triple double-quotes (""") prevent single-quotes, double-quotes, and double double-quotes from prematurely ending the string.
EDIT: I expanded the example to include things like \0 and \n. In a normal string (not a raw string) a \ (the escape character) signifies that the next character has special meaning. For example \n means "the newline character". If you literally wanted the character \ followed by n in your string you would have to write \\n, or just use a raw string instead, as I show in the example above.
You can also read about string literals in the Python documentation here:
For beginners: http://docs.python.org/tutorial/introduction.html#strings
Complex explanation: http://docs.python.org/reference/lexical_analysis.html#string-literals
Try triple quotes:
import re
tmp=""".*http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg.*"""
str="""http://www.mymaterialssite.com\'\,\'http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg','Model Photo'"""
x=re.match(tmp,str)
if x!=None:
print x.group()
Also you were missing the .* in the beginning of the pattern and at the end. I added that too.
if you use double quotes (which have the same meaning as the single ones, in Python), you don't have to escape at all.. (in this case). you can even use string literal without the starting r (you don't have any backslash there)
re.compile(".com','(.*?)','Model Photo")
Commas don't need to be escaped, and single quotes don't need to be escaped if you use double quotes to create the string:
>>> dummy=re.compile(r".com','(.*?)','Model Photo").search(temp).group(1)
>>> print dummy
http://images.mymaterials.com/images/steel-images/small/steel/steel800/steel800-2.jpg
Note that I also removed some unnecessary str() calls, and for future reference if you do ever need to escape single or double quotes (say your string contains both), use a backslash like this:
'.com\',\'(.*?)\',\'Model Photo'
As mykhal pointed out in comments, this doesn't work very nicely with regex because you can no longer use the raw string (r'...') literal. A better solution would be to use triple quoted strings as other answers suggested.