String literal

A string literal or anonymous string is a literal for a string value in the source code of a computer program. Modern programming languages commonly use a quoted sequence of characters, formally "bracketed delimiters", as in x = "foo", where , "foo" is a string literal with value foo. Methods such as escape sequences can be used to avoid the problem of delimiter collision (issues with brackets) and allow the delimiters to be embedded in a string. There are many alternate notations for specifying string literals especially in complicated cases. The exact notation depends on the programming language in question. Nevertheless, there are general guidelines that most modern programming languages follow.

Contents

• Syntax
• Bracketed delimiters
• Whitespace delimiters
• No delimiters
• Declarative notation
• Constructor functions
• Delimiter collision
• Doubling up
• Dual quoting
• Delimiter quoting
• Multiple quoting
• Constructor functions 2
• Escape sequences
• Nested escaping
• Raw strings
• Multiline string literals
• String literal concatenation
• Motivation
• Problems
• Different kinds of strings
• String interpolation
• Embedding source code in string literals
• See also
• Notes
• References
• External links

Syntax

 "Hi There!"

 ""

 'Hi There!'

"[^"]*"

 “Hi There!”  ‘Hi There!’  „Hi There!“  «Hi There!»

 {{Navbox  |name=Nulls  |title=[[wikt:Null|Nulls]] in [[computing]]  }}

-title:An example multi-line string in YAMLbody :|This is a multi-line string."special" metacharacters mayappear here. The extent of this string isrepresented by indentation.

%map=(red=>0x00f,blue=>0x0f0,green=>0xf00);

$y="x";$y=x;

35HAnexampleHollerithstringliteral

Delimiter collision

When using quoting, if one wishes to represent the delimiter itself in a string literal, one runs into the problem of delimiter collision. For example, if the delimiter is a double quote, one cannot simply represent a double quote itself by the literal """ as the second quote is interpreted as the end of the string literal, not as the value of the string, and similarly one cannot write "This is "in quotes", but invalid." as the middle quoted portion is instead interpreted as outside of quotes. There are various solutions, the most general-purpose of which is using escape sequences, such as "\"" or "This is \"in quotes\" and properly escaped.", but there are many other solutions.

Paired quotes, such as braces in Tcl, allow nested strings, such as {foo {bar} zork} but do not otherwise solve the problem of delimiter collision, since an unbalanced closing delimiter cannot simply be included, as in {}}.

'This Pascal string''contains two apostrophes'''

"I said, ""Can you hear me?"""

"This is John's apple."'I said, "Can you hear me?"'

'I said, "This is '+"John's"+' apple."'

'I said, "This is '"John's"' apple."'

qq^I said, "Can you hear me?"^qq@I said, "Can you hear me?"@qq§I said, "Can you hear me?"§

localls=[=[This notation can be used for Windows paths: local path = [[C:\Windows\Fonts]]]=]

"I said, "+CHR$(34)+"Can you hear me?"+CHR$(34)

charbuffer[32];snprintf(buffer,sizeofbuffer,"This is %cin quotes.%c",34,34);

Escape sequences

Escape sequences are a general technique for representing characters that are otherwise difficult to represent directly, including delimiters, nonprinting characters (such as backspaces), newlines, and whitespace characters (which are otherwise impossible to distinguish visually), and have a long history. They are accordingly widely used in string literals, and adding an escape sequence (either to a single character or throughout a string) is known as escaping.

One character is chosen as a prefix to give encodings for characters that are difficult or impossible to include directly. Most commonly this is backslash; in addition to other characters, a key point is that backslash itself can be encoded as a double backslash \\ and for delimited strings the delimiter itself can be encoded by escaping, say by \" for ". A regular expression for such escaped strings can be given as follows, as found in the ANSI C specification: ^{[1] [lower-alpha 1]}

"(\\.|[^\\"])*"

meaning "a quote; followed by zero or more of either an escaped character (backslash followed by something, possibly backslash or quote), or a non-escape, non-quote character; ending in a quote" – the only issue is distinguishing the terminating quote from a quote preceded by a backslash, which may itself be escaped. Multiple characters can follow the backslash, such as \uFFFF, depending on the escaping scheme.

An escaped string must then itself be lexically analyzed, converting the escaped string into the unescaped string that it represents. This is done during the evaluation phase of the overall lexing of the computer language: the evaluator of the lexer of the overall language executes its own lexer for escaped string literals.

Among other things, it must be possible to encode the character that normally terminates the string constant, plus there must be some way to specify the escape character itself. Escape sequences are not always pretty or easy to use, so many compilers also offer other means of solving the common problems. Escape sequences, however, solve every delimiter problem and most compilers interpret escape sequences. When an escape character is inside a string literal, it means "this is the start of the escape sequence". Every escape sequence specifies one character which is to be placed directly into the string. The actual number of characters required in an escape sequence varies. The escape character is on the top/left of the keyboard, but the editor will translate it, therefore it is not directly tapeable into a string. The backslash is used to represent the escape character in a string literal.

Many languages support the use of metacharacters inside string literals. Metacharacters have varying interpretations depending on the context and language, but are generally a kind of 'processing command' for representing printing or nonprinting characters.

For instance, in a C string literal, if the backslash is followed by a letter such as "b", "n" or "t", then this represents a nonprinting backspace, newline or tab character respectively. Or if the backslash is followed by 1-3 octal digits, then this sequence is interpreted as representing the arbitrary code unit with the specified value in the literal's encoding (for example, the corresponding ASCII code for an ASCII literal). This was later extended to allow more modern hexadecimal character code notation:

"I said,\t\t\x22Can you hear me?\x22\n"

Note: Not all sequences in the list are supported by all parsers, and there may be other escape sequences which are not in the list.

"The Windows path is C:\\Foo\\Bar\\Baz\\"@"The Windows path is C:\Foo\Bar\Baz\"

<![CDATA[  if (path!=null && depth<2) { add(path); }  ]]>

Multiline string literals

In many languages, string literals can contain literal newlines, spanning several lines. Alternatively, newlines can be escaped, most often as \n. For example:

echo'foobar'

and

echo-e"foo\nbar"

are both valid bash, producing:

foo bar

Languages that allow literal newlines include bash, Lua, Perl, PHP, R, and Tcl. In some other languages string literals cannot include newlines.

Two issues with multiline string literals are leading and trailing newlines, and indentation. If the initial or final delimiters are on separate lines, there are extra newlines, while if they are not, the delimiter makes the string harder to read, particularly for the first line, which is often indented differently from the rest. Further, the literal must be unindented, as leading whitespace is preserved – this breaks the flow of the code if the literal occurs within indented code.

The most common solution for these problems is here document-style string literals. Formally speaking, a here document is not a string literal, but instead a stream literal or file literal. These originate in shell scripts and allow a literal to be fed as input to an external command. The opening delimiter is <<END where END can be any word, and the closing delimiter is END on a line by itself, serving as a content boundary – the << is due to redirecting stdin from the literal. Due to the delimiter being arbitrary, these also avoid the problem of delimiter collision. These also allow initial tabs to be stripped via the variant syntax <<-END though leading spaces are not stripped. The same syntax has since been adopted for multiline string literals in a number of languages, most notably Perl, and are also referred to as here documents, and retain the syntax, despite being strings and not involving redirection. As with other string literals, these can sometimes have different behavior specified, such as variable interpolation.

Python, whose usual string literals do not allow literal newlines, instead has a special form of string, designed for multiline literals, called triple quoting. These use a tripled delimiter, either ''' or """. These literals are especially used for inline documentation, known as docstrings.

Tcl allows literal newlines in strings and has no special syntax to assist with multiline strings, though delimiters can be placed on lines by themselves and leading and trailing newlines stripped via string trim, while string map can be used to strip indentation.

String literal concatenation

Wikifunctions has a function related to this topic .

A few languages provide string literal concatenation, where adjacent string literals are implicitly joined into a single literal at compile time. This is a feature of C, ^{[7] [8]} C++, ^[9] D, ^[10] Ruby, ^[11] and Python, ^[12] which copied it from C. ^[13] Notably, this concatenation happens at compile time, during lexical analysis (as a phase following initial tokenization), and is contrasted with both run time string concatenation (generally with the + operator) ^[14] and concatenation during constant folding, which occurs at compile time, but in a later phase (after phrase analysis or "parsing"). Most languages, such as C#, Java ^[15] and Perl, do not support implicit string literal concatenation, and instead require explicit concatenation, such as with the + operator (this is also possible in D and Python, but illegal in C/C++ – see below); in this case concatenation may happen at compile time, via constant folding, or may be deferred to run time.

char*s="hello, ""world";printf("hello, ""world");

char*s="hello, "+"world";printf("hello, "+"world");

char*file_and_message=__FILE__": message";

char*file_and_message="a.c"": message";

char*file_and_message="a.c: message";

#define STRINGIFY(x) #x#define TOSTRING(x) STRINGIFY(x)#define AT __FILE__ ":" TOSTRING(__LINE__)

re.compile("[A-Za-z_]"# letter or underscore"[A-Za-z0-9_]*"# letter, digit or underscore)

l=['foo','bar''zork']

Different kinds of strings

Some languages provide more than one kind of literal, which have different behavior. This is particularly used to indicate raw strings (no escaping), or to disable or enable variable interpolation, but has other uses, such as distinguishing character sets. Most often this is done by changing the quoting character or adding a prefix or suffix. This is comparable to prefixes and suffixes to integer literals, such as to indicate hexadecimal numbers or long integers.

One of the oldest examples is in shell scripts, where single quotes indicate a raw string or "literal string", while double quotes have escape sequences and variable interpolation.

For example, in Python, raw strings are preceded by an r or R – compare 'C:\\Windows' with r'C:\Windows' (though, a Python raw string cannot end in an odd number of backslashes). Python 2 also distinguishes two types of strings: 8-bit ASCII ("bytes") strings (the default), explicitly indicated with a b or B prefix, and Unicode strings, indicated with a u or U prefix. ^[25] while in Python 3 strings are Unicode by default and bytes are a separate bytes type that when initialized with quotes must be prefixed with a b.

C#'s notation for raw strings is called @-quoting.

@"C:\Foo\Bar\Baz\"

While this disables escaping, it allows double-up quotes, which allow one to represent quotes within the string:

@"I said, ""Hello there."""

C++11 allows raw strings, unicode strings (UTF-8, UTF-16, and UTF-32), and wide character strings, determined by prefixes. It also adds literals for the existing C++ string, which is generally preferred to the existing C-style strings.

In Tcl, brace-delimited strings are literal, while quote-delimited strings have escaping and interpolation.

Perl has a wide variety of strings, which are more formally considered operators, and are known as quote and quote-like operators. These include both a usual syntax (fixed delimiters) and a generic syntax, which allows a choice of delimiters; these include: ^[26]

''""``//m//qr//s///y///q{}qq{}qx{}qw{}m{}qr{}s{}{}tr{}{}y{}{}

REXX uses suffix characters to specify characters or strings using their hexadecimal or binary code. E.g.,

'20'x "0010 0000"b "00100000"b 

all yield the space character, avoiding the function call X2C(20).

String interpolation

In some languages, string literals may contain placeholders referring to variables or expressions in the current context, which are evaluated (usually at run time). This is referred to as variable interpolation, or more generally string interpolation. Languages that support interpolation generally distinguish strings literals that are interpolated from ones that are not. For example, in sh-compatible Unix shells (as well as Perl and Ruby), double-quoted (quotation-delimited, ") strings are interpolated, while single-quoted (apostrophe-delimited, ') strings are not. Non-interpolated string literals are sometimes referred to as "raw strings", but this is distinct from "raw string" in the sense of escaping. For example, in Python, a string prefixed with r or R has no escaping or interpolation, a normal string (no prefix) has escaping but no interpolation, and a string prefixed with f or F has escaping and interpolation.

For example, the following Perl code:

$name="Nancy";$greeting="Hello World";print"$name said $greeting to the crowd of people.";

produces the output:

Nancy said Hello World to the crowd of people.

In this case, the metacharacter character ($) (not to be confused with the sigil in the variable assignment statement) is interpreted to indicate variable interpolation, and requires some escaping if it needs to be outputted literally.

This should be contrasted with the printf function, which produces the same output using notation such as:

printf"%s said %s to the crowd of people.",$name,$greeting;

but does not perform interpolation: the %s is a placeholder in a printf format string, but the variables themselves are outside the string.

This is contrasted with "raw" strings:

print'$name said $greeting to the crowd of people.';

which produce output like:

$name said $greeting to the crowd of people.

Here the $ characters are not metacharacters, and are not interpreted to have any meaning other than plain text.

Embedding source code in string literals

Languages that lack flexibility in specifying string literals make it particularly cumbersome to write programming code that generates other programming code. This is particularly true when the generation language is the same or similar to the output language.

For example:

• writing code to produce quines
• generating an output language from within a web template;
• using XSLT to generate XSLT, or SQL to generate more SQL
• generating a PostScript representation of a document for printing purposes, from within a document-processing application written in C or some other language.

Nevertheless, some languages are particularly well-adapted to produce this sort of self-similar output, especially those that support multiple options for avoiding delimiter collision.

Using string literals as code that generates other code may have adverse security implications, especially if the output is based at least partially on untrusted user input. This is particularly acute in the case of Web-based applications, where malicious users can take advantage of such weaknesses to subvert the operation of the application, for example by mounting an SQL injection attack.

See also

• Character literal
• XML Literals
• Sigil (computer programming)

Notes

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References

External links

• Literals In Programming