究竟什么是“原始字符串正则表达式”以及如何使用它？

Question

From the python documentation on regex , regarding the '\' character:

The solution is to use Python's raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with 'r' . So r"\n" is a two-character string containing '\' and 'n' , while "\n" is a one-character string containing a newline. Usually patterns will be expressed in Python code using this raw string notation.

What is this raw string notation? If you use a raw string format, does that mean "*" is taken as aa literal character rather than a zero-or-more indicator? That obviously can't be right, or else regex would completely lose its power. But then if it's a raw string, how does it recognize newline characters if "\n" is literally a backslash and an "n" ?

I don't follow.

Edit for bounty:

I'm trying to understand how a raw string regex matches newlines, tabs, and character sets, eg \w for words or \d for digits or all whatnot, if raw string patterns don't recognize backslashes as anything more than ordinary characters. I could really use some good examples.

Answer 1

Zarkonnen's response does answer your question, but not directly. Let me try to be more direct, and see if I can grab the bounty from Zarkonnen.

You will perhaps find this easier to understand if you stop using the terms "raw string regex" and "raw string patterns". These terms conflate two separate concepts: the representations of a particular string in Python source code, and what regular expression that string represents.

In fact, it's helpful to think of these as two different programming languages, each with their own syntax. The Python language has source code that, among other things, builds strings with certain contents, and calls the regular expression system. The regular expression system has source code that resides in string objects, and matches strings. Both languages use backslash as an escape character.

First, understand that a string is a sequence of characters (ie bytes or Unicode code points; the distinction doesn't much matter here). There are many ways to represent a string in Python source code. A raw string is simply one of these representations. If two representations result in the same sequence of characters, they produce equivalent behaviour.

Imagine a 2-character string, consisting of the backslash character followed by the n character. If you know that the character value for backslash is 92, and for n is 110, then this expression generates our string:

s = chr(92)+chr(110)
print len(s), s

2 \n

The conventional Python string notation "\n" does not generate this string. Instead it generates a one-character string with a newline character. The Python docs 2.4.1. String literals say, "The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character."

s = "\n"
print len(s), s

1 
 

(Note that the newline isn't visible in this example, but if you look carefully, you'll see a blank line after the "1".)

To get our two-character string, we have to use another backslash character to escape the special meaning of the original backslash character:

s = "\\n"
print len(s), s

2 \n

What if you want to represent strings that have many backslash characters in them? Python docs 2.4.1. String literals continue, "String literals may optionally be prefixed with a letter 'r' or 'R'; such strings are called raw strings and use different rules for interpreting backslash escape sequences." Here is our two-character string, using raw string representation:

s = r"\n"
print len(s), s

2 \n

So we have three different string representations, all giving the same string, or sequence of characters:

print chr(92)+chr(110) == "\\n" == r"\n"
True

Now, let's turn to regular expressions. The Python docs, 7.2. re — Regular expression operations says, "Regular expressions use the backslash character ('\') to indicate special forms or to allow special characters to be used without invoking their special meaning. This collides with Python's usage of the same character for the same purpose in string literals..."

If you want a Python regular expression object which matches a newline character, then you need a 2-character string, consisting of the backslash character followed by the n character. The following lines of code all set prog to a regular expression object which recognises a newline character:

prog = re.compile(chr(92)+chr(110))
prog = re.compile("\\n")
prog = re.compile(r"\n")

So why is it that "Usually patterns will be expressed in Python code using this raw string notation." ? Because regular expressions are frequently static strings, which are conveniently represented as string literals. And from the different string literal notations available, raw strings are a convenient choice, when the regular expression includes a backslash character.

Questions

Q : what about the expression re.compile(r"\s\tWord") ? A : It's easier to understand by separating the string from the regular expression compilation, and understanding them separately.

s = r"\s\tWord"
prog = re.compile(s)

The string s contains eight characters: a backslash , an s , a backslash , a t , and then four characters Word .

Q : What happens to the tab and space characters? A : At the Python language level, string s doesn't have tab and space character. It starts with four characters: backslash , s , backslash , t . The regular expression system, meanwhile, treats that string as source code in the regular expression language, where it means "match a string consisting of a whitespace character, a tab character, and the four characters Word .

Q : How do you match those if that's being treated as backlash-s and backslash-t? A : Maybe the question is clearer if the words 'you' and 'that' are made more specific: how does the regular expression system match the expressions backlash-s and backslash-t? As 'any whitespace character' and as ' tab character'.

Q : Or what if you have the 3-character string backslash-n-newline? A : In the Python language, the 3-character string backslash-n-newline can be represented as conventional string "\\n\n" , or raw plus conventional string r"\n" "\n" , or in other ways. The regular expression system matches the 3-character string backslash-n-newline when it finds any two consecutive newline characters.

NB All examples and document references are to Python 2.7.

Update : Incorporated clarifications from answers of @Vladislav Zorov and @m.buettner, and from follow-up question of @Aerovistae.

Answer 2

Most of these questions have a lot of words in them and maybe it's hard to find the answer to your specific question.

If you use a regular string and you pass in a pattern like "\t" to the RegEx parser, Python will translate that literal into a buffer with the tab byte in it (0x09).

If you use a raw string and you pass in a pattern like r"\t" to the RegEx parser, Python does not do any interpretation, and it creates a buffer with two bytes in it: '\', and 't'. (0x5c, 0x74).

The RegEx parser knows what to do with the sequence '\t' -- it matches that against a tab. It also knows what to do with the 0x09 character -- that also matches a tab. For the most part, the results will be indistinguishable.

So the key to understanding what's happening is recognizing that there are two parsers being employed here. The first one is the Python parser, and it translates your string literal (or raw string literal) into a sequence of bytes. The second one is Python's regular expression parser, and it converts a sequence of bytes into a compiled regular expression.

Answer 3

The issue with using a normal string to write regexes that contain a \ is that you end up having to write \\ for every \ . So the string literals "stuff\\things" and r"stuff\things" produce the same string. This gets especially useful if you want to write a regular expression that matches against backslashes.

Using normal strings, a regexp that matches the string \ would be "\\\\" !

Why? Because we have to escape \ twice: once for the regular expression syntax, and once for the string syntax.

You can use triple quotes to include newlines, like this:

r'''stuff\
things'''

Note that usually, python would treat \ -newline as a line continuation, but this is not the case in raw strings. Also note that backslashes still escape quotes in raw strings, but are left in themselves. So the raw string literal r"\"" produces the string \" . This means you can't end a raw string literal with a backslash.

See the lexical analysis section of the Python documentation for more information.

Answer 4

You seem to be struggling with the idea that a RegEx isn't part of Python, but instead a different programming language with its own parser and compiler. Raw strings help you get the "source code" of a RegEx safely to the RegEx parser, which will then assign meaning to character sequences like \d , \w , \n , etc...

The issue exists because Python and RegExps use \ as escape character, which is, by the way, a coincidence - there are languages with other escape characters (like "`n" for a newline, but even there you have to use "\n" in RegExps). The advantage is that you don't need to differentiate between raw and non-raw strings in these languages, they won't both try to convert the text and butcher it, because they react to different escape sequences.

Answer 5

The relevant Python manual section ("String and Bytes literals") has a clear explanation of raw string literals:

Both string and bytes literals may optionally be prefixed with a letter 'r' or 'R'; such strings are called raw strings and treat backslashes as literal characters. As a result, in string literals, '\U' and '\u' escapes in raw strings are not treated specially. Given that Python 2.x's raw unicode literals behave differently than Python 3.x's the 'ur' syntax is not supported.

New in version 3.3: The 'rb' prefix of raw bytes literals has been added as a synonym of 'br'.

New in version 3.3: Support for the unicode legacy literal (u'value') was reintroduced to simplify the maintenance of dual Python 2.x and 3.x codebases. See PEP 414 for more information.

In triple-quoted strings, unescaped newlines and quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the string. (A “quote” is the character used to open the string, ie either ' or ".)

Unless an 'r' or 'R' prefix is present, escape sequences in strings are interpreted according to rules similar to those used by Standard C. The recognized escape sequences are:

Escape Sequence Meaning Notes

\newline Backslash and newline ignored
\ Backslash ()
\' Single quote (')
\" Double quote (")
\a ASCII Bell (BEL)
\b ASCII Backspace (BS)
\f ASCII Formfeed (FF)
\n ASCII Linefeed (LF)
\r ASCII Carriage Return (CR)
\t ASCII Horizontal Tab (TAB) \v ASCII Vertical Tab (VT)
\ooo Character with octal value ooo (1,3)
\xhh Character with hex value hh (2,3)

Escape sequences only recognized in string literals are:

Escape Sequence Meaning Notes \N{name} Character named name in the Unicode database (4) \uxxxx Character with 16-bit hex value xxxx (5) \Uxxxxxxxx Character with 32-bit hex value xxxxxxxx (6)

Notes:

1. As in Standard C, up to three octal digits are accepted.

2. Unlike in Standard C, exactly two hex digits are required.

3. In a bytes literal, hexadecimal and octal escapes denote the byte with the given value. In a string literal, these escapes denote a Unicode character with the given value.

4. Changed in version 3.3: Support for name aliases [1] has been added.

5. Individual code units which form parts of a surrogate pair can be encoded using this escape sequence. Exactly four hex digits are required.

6. Any Unicode character can be encoded this way, but characters outside the Basic Multilingual Plane (BMP) will be encoded using a surrogate pair if Python is compiled to use 16-bit code units (the default). Exactly eight hex digits are required.

Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, ie, the backslash is left in the string. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken.) It is also important to note that the escape sequences only recognized in string literals fall into the category of unrecognized escapes for bytes literals.

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). 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.

Answer 6

\n is an Escape Sequence in Python

\w is a Special Sequence in (Python) Regex

They look like they are in the same family but they are not. Raw string notation will affect Escape Sequences but not Regex Special Sequences.

For more about Escape Sequences search for "\newline" https://docs.python.org/3/reference/lexical_analysis.html

For more about Special Sequences: search for "\number" https://docs.python.org/3/library/re.html

Answer 7

raw string does not affect special sequences in python regex such as \w, \d. It only affects escape sequences such as \n. So most of the time it doesn't matter we write r in front or not.

I think that is the answer most beginners are looking for.