Strings

Strings

Strings are finite sequences of characters. Of course, the real trouble comes when one asks what a character is. The characters that English speakers are familiar with are the letters A, B, C, etc., together with numerals and common punctuation symbols. These characters are standardized together with a mapping to integer values between 0 and 127 by the ASCII standard. There are, of course, many other characters used in non-English languages, including variants of the ASCII characters with accents and other modifications, related scripts such as Cyrillic and Greek, and scripts completely unrelated to ASCII and English, including Arabic, Chinese, Hebrew, Hindi, Japanese, and Korean. The Unicode standard tackles the complexities of what exactly a character is, and is generally accepted as the definitive standard addressing this problem. Depending on your needs, you can either ignore these complexities entirely and just pretend that only ASCII characters exist, or you can write code that can handle any of the characters or encodings that one may encounter when handling non-ASCII text. Julia makes dealing with plain ASCII text simple and efficient, and handling Unicode is as simple and efficient as possible. In particular, you can write C-style string code to process ASCII strings, and they will work as expected, both in terms of performance and semantics. If such code encounters non-ASCII text, it will gracefully fail with a clear error message, rather than silently introducing corrupt results. When this happens, modifying the code to handle non-ASCII data is straightforward.

There are a few noteworthy high-level features about Julia's strings:

Characters

A Char value represents a single character: it is just a 32-bit primitive type with a special literal representation and appropriate arithmetic behaviors, whose numeric value is interpreted as a Unicode code point. Here is how Char values are input and shown:

julia> 'x'
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)

julia> typeof(ans)
Char

You can convert a Char to its integer value, i.e. code point, easily:

julia> Int('x')
120

julia> typeof(ans)
Int64

On 32-bit architectures, typeof(ans) will be Int32. You can convert an integer value back to a Char just as easily:

julia> Char(120)
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)

Not all integer values are valid Unicode code points, but for performance, the Char() conversion does not check that every character value is valid. If you want to check that each converted value is a valid code point, use the isvalid() function:

julia> Char(0x110000)
'\U110000': Unicode U+110000 (category Cn: Other, not assigned)

julia> isvalid(Char, 0x110000)
false

As of this writing, the valid Unicode code points are U+00 through U+d7ff and U+e000 through U+10ffff. These have not all been assigned intelligible meanings yet, nor are they necessarily interpretable by applications, but all of these values are considered to be valid Unicode characters.

You can input any Unicode character in single quotes using \u followed by up to four hexadecimal digits or \U followed by up to eight hexadecimal digits (the longest valid value only requires six):

julia> '\u0'
'\0': ASCII/Unicode U+0000 (category Cc: Other, control)

julia> '\u78'
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)

julia> '\u2200'
'∀': Unicode U+2200 (category Sm: Symbol, math)

julia> '\U10ffff'
'\U10ffff': Unicode U+10ffff (category Cn: Other, not assigned)

Julia uses your system's locale and language settings to determine which characters can be printed as-is and which must be output using the generic, escaped \u or \U input forms. In addition to these Unicode escape forms, all of C's traditional escaped input forms can also be used:

julia> Int('\0')
0

julia> Int('\t')
9

julia> Int('\n')
10

julia> Int('\e')
27

julia> Int('\x7f')
127

julia> Int('\177')
127

julia> Int('\xff')
255

You can do comparisons and a limited amount of arithmetic with Char values:

julia> 'A' < 'a'
true

julia> 'A' <= 'a' <= 'Z'
false

julia> 'A' <= 'X' <= 'Z'
true

julia> 'x' - 'a'
23

julia> 'A' + 1
'B': ASCII/Unicode U+0042 (category Lu: Letter, uppercase)

String Basics

String literals are delimited by double quotes or triple double quotes:

julia> str = "Hello, world.\n"
"Hello, world.\n"

julia> """Contains "quote" characters"""
"Contains \"quote\" characters"

If you want to extract a character from a string, you index into it:

julia> str[1]
'H': ASCII/Unicode U+0048 (category Lu: Letter, uppercase)

julia> str[6]
',': ASCII/Unicode U+002c (category Po: Punctuation, other)

julia> str[end]
'\n': ASCII/Unicode U+000a (category Cc: Other, control)

All indexing in Julia is 1-based: the first element of any integer-indexed object is found at index 1. (As we will see below, this does not necessarily mean that the last element is found at index n, where n is the length of the string.)

In any indexing expression, the keyword end can be used as a shorthand for the last index (computed by endof(str)). You can perform arithmetic and other operations with end, just like a normal value:

julia> str[end-1]
'.': ASCII/Unicode U+002e (category Po: Punctuation, other)

julia> str[end÷2]
' ': ASCII/Unicode U+0020 (category Zs: Separator, space)

Using an index less than 1 or greater than end raises an error:

julia> str[0]
ERROR: BoundsError: attempt to access "Hello, world.\n"
  at index [0]
[...]

julia> str[end+1]
ERROR: BoundsError: attempt to access "Hello, world.\n"
  at index [15]
[...]

You can also extract a substring using range indexing:

julia> str[4:9]
"lo, wo"

Notice that the expressions str[k] and str[k:k] do not give the same result:

julia> str[6]
',': ASCII/Unicode U+002c (category Po: Punctuation, other)

julia> str[6:6]
","

The former is a single character value of type Char, while the latter is a string value that happens to contain only a single character. In Julia these are very different things.

Unicode and UTF-8

Julia fully supports Unicode characters and strings. As discussed above, in character literals, Unicode code points can be represented using Unicode \u and \U escape sequences, as well as all the standard C escape sequences. These can likewise be used to write string literals:

julia> s = "\u2200 x \u2203 y"
"∀ x ∃ y"

Whether these Unicode characters are displayed as escapes or shown as special characters depends on your terminal's locale settings and its support for Unicode. String literals are encoded using the UTF-8 encoding. UTF-8 is a variable-width encoding, meaning that not all characters are encoded in the same number of bytes. In UTF-8, ASCII characters – i.e. those with code points less than 0x80 (128) – are encoded as they are in ASCII, using a single byte, while code points 0x80 and above are encoded using multiple bytes – up to four per character. This means that not every byte index into a UTF-8 string is necessarily a valid index for a character. If you index into a string at such an invalid byte index, an error is thrown:

julia> s[1]
'∀': Unicode U+2200 (category Sm: Symbol, math)

julia> s[2]
ERROR: UnicodeError: invalid character index
[...]

julia> s[3]
ERROR: UnicodeError: invalid character index
[...]

julia> s[4]
' ': ASCII/Unicode U+0020 (category Zs: Separator, space)

In this case, the character ∀ is a three-byte character, so the indices 2 and 3 are invalid and the next character's index is 4; this next valid index can be computed by nextind(s,1), and the next index after that by nextind(s,4) and so on.

Because of variable-length encodings, the number of characters in a string (given by length(s)) is not always the same as the last index. If you iterate through the indices 1 through endof(s) and index into s, the sequence of characters returned when errors aren't thrown is the sequence of characters comprising the string s. Thus we have the identity that length(s) <= endof(s), since each character in a string must have its own index. The following is an inefficient and verbose way to iterate through the characters of s:

julia> for i = 1:endof(s)
           try
               println(s[i])
           catch
               # ignore the index error
           end
       end
∀

x

∃

y

The blank lines actually have spaces on them. Fortunately, the above awkward idiom is unnecessary for iterating through the characters in a string, since you can just use the string as an iterable object, no exception handling required:

julia> for c in s
           println(c)
       end
∀

x

∃

y

Julia uses the UTF-8 encoding by default, and support for new encodings can be added by packages. For example, the LegacyStrings.jl package implements UTF16String and UTF32String types. Additional discussion of other encodings and how to implement support for them is beyond the scope of this document for the time being. For further discussion of UTF-8 encoding issues, see the section below on byte array literals. The transcode() function is provided to convert data between the various UTF-xx encodings, primarily for working with external data and libraries.

Concatenation

One of the most common and useful string operations is concatenation:

julia> greet = "Hello"
"Hello"

julia> whom = "world"
"world"

julia> string(greet, ", ", whom, ".\n")
"Hello, world.\n"

Julia also provides * for string concatenation:

julia> greet * ", " * whom * ".\n"
"Hello, world.\n"

While * may seem like a surprising choice to users of languages that provide + for string concatenation, this use of * has precedent in mathematics, particularly in abstract algebra.

In mathematics, + usually denotes a commutative operation, where the order of the operands does not matter. An example of this is matrix addition, where A + B == B + A for any matrices A and B that have the same shape. In contrast, * typically denotes a noncommutative operation, where the order of the operands does matter. An example of this is matrix multiplication, where in general A * B != B * A. As with matrix multiplication, string concatenation is noncommutative: greet * whom != whom * greet. As such, * is a more natural choice for an infix string concatenation operator, consistent with common mathematical use.

More precisely, the set of all finite-length strings S together with the string concatenation operator * forms a free monoid (S, *). The identity element of this set is the empty string, "". Whenever a free monoid is not commutative, the operation is typically represented as \cdot, *, or a similar symbol, rather than +, which as stated usually implies commutativity.

Interpolation

Constructing strings using concatenation can become a bit cumbersome, however. To reduce the need for these verbose calls to string() or repeated multiplications, Julia allows interpolation into string literals using $, as in Perl:

julia> "$greet, $whom.\n"
"Hello, world.\n"

This is more readable and convenient and equivalent to the above string concatenation – the system rewrites this apparent single string literal into a concatenation of string literals with variables.

The shortest complete expression after the $ is taken as the expression whose value is to be interpolated into the string. Thus, you can interpolate any expression into a string using parentheses:

julia> "1 + 2 = $(1 + 2)"
"1 + 2 = 3"

Both concatenation and string interpolation call string() to convert objects into string form. Most non-AbstractString objects are converted to strings closely corresponding to how they are entered as literal expressions:

julia> v = [1,2,3]
3-element Array{Int64,1}:
 1
 2
 3

julia> "v: $v"
"v: [1, 2, 3]"

string() is the identity for AbstractString and Char values, so these are interpolated into strings as themselves, unquoted and unescaped:

julia> c = 'x'
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)

julia> "hi, $c"
"hi, x"

To include a literal $ in a string literal, escape it with a backslash:

julia> print("I have \$100 in my account.\n")
I have $100 in my account.

Triple-Quoted String Literals

When strings are created using triple-quotes ("""...""") they have some special behavior that can be useful for creating longer blocks of text. First, if the opening """ is followed by a newline, the newline is stripped from the resulting string.

"""hello"""

is equivalent to

"""
hello"""

but

"""

hello"""

will contain a literal newline at the beginning. Trailing whitespace is left unaltered. They can contain " symbols without escaping. Triple-quoted strings are also dedented to the level of the least-indented line. This is useful for defining strings within code that is indented. For example:

julia> str = """
           Hello,
           world.
         """
"  Hello,\n  world.\n"

In this case the final (empty) line before the closing """ sets the indentation level.

Note that line breaks in literal strings, whether single- or triple-quoted, result in a newline (LF) character \n in the string, even if your editor uses a carriage return \r (CR) or CRLF combination to end lines. To include a CR in a string, use an explicit escape \r; for example, you can enter the literal string "a CRLF line ending\r\n".

Common Operations

You can lexicographically compare strings using the standard comparison operators:

julia> "abracadabra" < "xylophone"
true

julia> "abracadabra" == "xylophone"
false

julia> "Hello, world." != "Goodbye, world."
true

julia> "1 + 2 = 3" == "1 + 2 = $(1 + 2)"
true

You can search for the index of a particular character using the search() function:

julia> search("xylophone", 'x')
1

julia> search("xylophone", 'p')
5

julia> search("xylophone", 'z')
0

You can start the search for a character at a given offset by providing a third argument:

julia> search("xylophone", 'o')
4

julia> search("xylophone", 'o', 5)
7

julia> search("xylophone", 'o', 8)
0

You can use the contains() function to check if a substring is contained in a string:

julia> contains("Hello, world.", "world")
true

julia> contains("Xylophon", "o")
true

julia> contains("Xylophon", "a")
false

julia> contains("Xylophon", 'o')
ERROR: MethodError: no method matching contains(::String, ::Char)
Closest candidates are:
  contains(!Matched::Function, ::Any, !Matched::Any) at reduce.jl:664
  contains(::AbstractString, !Matched::AbstractString) at strings/search.jl:378

The last error is because 'o' is a character literal, and contains() is a generic function that looks for subsequences. To look for an element in a sequence, you must use in() instead.

Two other handy string functions are repeat() and join():

julia> repeat(".:Z:.", 10)
".:Z:..:Z:..:Z:..:Z:..:Z:..:Z:..:Z:..:Z:..:Z:..:Z:."

julia> join(["apples", "bananas", "pineapples"], ", ", " and ")
"apples, bananas and pineapples"

Some other useful functions include:

Non-Standard String Literals

There are situations when you want to construct a string or use string semantics, but the behavior of the standard string construct is not quite what is needed. For these kinds of situations, Julia provides non-standard string literals. A non-standard string literal looks like a regular double-quoted string literal, but is immediately prefixed by an identifier, and doesn't behave quite like a normal string literal. Regular expressions, byte array literals and version number literals, as described below, are some examples of non-standard string literals. Other examples are given in the Metaprogramming section.

Regular Expressions

Julia has Perl-compatible regular expressions (regexes), as provided by the PCRE library. Regular expressions are related to strings in two ways: the obvious connection is that regular expressions are used to find regular patterns in strings; the other connection is that regular expressions are themselves input as strings, which are parsed into a state machine that can be used to efficiently search for patterns in strings. In Julia, regular expressions are input using non-standard string literals prefixed with various identifiers beginning with r. The most basic regular expression literal without any options turned on just uses r"...":

julia> r"^\s*(?:#|$)"
r"^\s*(?:#|$)"

julia> typeof(ans)
Regex

To check if a regex matches a string, use ismatch():

julia> ismatch(r"^\s*(?:#|$)", "not a comment")
false

julia> ismatch(r"^\s*(?:#|$)", "# a comment")
true

As one can see here, ismatch() simply returns true or false, indicating whether the given regex matches the string or not. Commonly, however, one wants to know not just whether a string matched, but also how it matched. To capture this information about a match, use the match() function instead:

julia> match(r"^\s*(?:#|$)", "not a comment")

julia> match(r"^\s*(?:#|$)", "# a comment")
RegexMatch("#")

If the regular expression does not match the given string, match() returns nothing – a special value that does not print anything at the interactive prompt. Other than not printing, it is a completely normal value and you can test for it programmatically:

m = match(r"^\s*(?:#|$)", line)
if m === nothing
    println("not a comment")
else
    println("blank or comment")
end

If a regular expression does match, the value returned by match() is a RegexMatch object. These objects record how the expression matches, including the substring that the pattern matches and any captured substrings, if there are any. This example only captures the portion of the substring that matches, but perhaps we want to capture any non-blank text after the comment character. We could do the following:

julia> m = match(r"^\s*(?:#\s*(.*?)\s*$|$)", "# a comment ")
RegexMatch("# a comment ", 1="a comment")

When calling match(), you have the option to specify an index at which to start the search. For example:

julia> m = match(r"[0-9]","aaaa1aaaa2aaaa3",1)
RegexMatch("1")

julia> m = match(r"[0-9]","aaaa1aaaa2aaaa3",6)
RegexMatch("2")

julia> m = match(r"[0-9]","aaaa1aaaa2aaaa3",11)
RegexMatch("3")

You can extract the following info from a RegexMatch object:

For when a capture doesn't match, instead of a substring, m.captures contains nothing in that position, and m.offsets has a zero offset (recall that indices in Julia are 1-based, so a zero offset into a string is invalid). Here is a pair of somewhat contrived examples:

julia> m = match(r"(a|b)(c)?(d)", "acd")
RegexMatch("acd", 1="a", 2="c", 3="d")

julia> m.match
"acd"

julia> m.captures
3-element Array{Union{SubString{String}, Void},1}:
 "a"
 "c"
 "d"

julia> m.offset
1

julia> m.offsets
3-element Array{Int64,1}:
 1
 2
 3

julia> m = match(r"(a|b)(c)?(d)", "ad")
RegexMatch("ad", 1="a", 2=nothing, 3="d")

julia> m.match
"ad"

julia> m.captures
3-element Array{Union{SubString{String}, Void},1}:
 "a"
 nothing
 "d"

julia> m.offset
1

julia> m.offsets
3-element Array{Int64,1}:
 1
 0
 2

It is convenient to have captures returned as an array so that one can use destructuring syntax to bind them to local variables:

julia> first, second, third = m.captures; first
"a"

Captures can also be accessed by indexing the RegexMatch object with the number or name of the capture group:

julia> m=match(r"(?<hour>\d+):(?<minute>\d+)","12:45")
RegexMatch("12:45", hour="12", minute="45")

julia> m[:minute]
"45"

julia> m[2]
"45"

Captures can be referenced in a substitution string when using replace() by using \n to refer to the nth capture group and prefixing the substitution string with s. Capture group 0 refers to the entire match object. Named capture groups can be referenced in the substitution with g<groupname>. For example:

julia> replace("first second", r"(\w+) (?<agroup>\w+)", s"\g<agroup> \1")
"second first"

Numbered capture groups can also be referenced as \g<n> for disambiguation, as in:

julia> replace("a", r".", s"\g<0>1")
"a1"

You can modify the behavior of regular expressions by some combination of the flags i, m, s, and x after the closing double quote mark. These flags have the same meaning as they do in Perl, as explained in this excerpt from the perlre manpage:

i   Do case-insensitive pattern matching.

    If locale matching rules are in effect, the case map is taken
    from the current locale for code points less than 255, and
    from Unicode rules for larger code points. However, matches
    that would cross the Unicode rules/non-Unicode rules boundary
    (ords 255/256) will not succeed.

m   Treat string as multiple lines.  That is, change "^" and "$"
    from matching the start or end of the string to matching the
    start or end of any line anywhere within the string.

s   Treat string as single line.  That is, change "." to match any
    character whatsoever, even a newline, which normally it would
    not match.

    Used together, as r""ms, they let the "." match any character
    whatsoever, while still allowing "^" and "$" to match,
    respectively, just after and just before newlines within the
    string.

x   Tells the regular expression parser to ignore most whitespace
    that is neither backslashed nor within a character class. You
    can use this to break up your regular expression into
    (slightly) more readable parts. The '#' character is also
    treated as a metacharacter introducing a comment, just as in
    ordinary code.

For example, the following regex has all three flags turned on:

julia> r"a+.*b+.*?d$"ism
r"a+.*b+.*?d$"ims

julia> match(r"a+.*b+.*?d$"ism, "Goodbye,\nOh, angry,\nBad world\n")
RegexMatch("angry,\nBad world")

Triple-quoted regex strings, of the form r"""...""", are also supported (and may be convenient for regular expressions containing quotation marks or newlines).

Byte Array Literals

Another useful non-standard string literal is the byte-array string literal: b"...". This form lets you use string notation to express literal byte arrays – i.e. arrays of UInt8 values. The rules for byte array literals are the following:

There is some overlap between these rules since the behavior of \x and octal escapes less than 0x80 (128) are covered by both of the first two rules, but here these rules agree. Together, these rules allow one to easily use ASCII characters, arbitrary byte values, and UTF-8 sequences to produce arrays of bytes. Here is an example using all three:

julia> b"DATA\xff\u2200"
8-element Array{UInt8,1}:
 0x44
 0x41
 0x54
 0x41
 0xff
 0xe2
 0x88
 0x80

The ASCII string "DATA" corresponds to the bytes 68, 65, 84, 65. \xff produces the single byte 255. The Unicode escape \u2200 is encoded in UTF-8 as the three bytes 226, 136, 128. Note that the resulting byte array does not correspond to a valid UTF-8 string – if you try to use this as a regular string literal, you will get a syntax error:

julia> "DATA\xff\u2200"
ERROR: syntax: invalid UTF-8 sequence

Also observe the significant distinction between \xff and \uff: the former escape sequence encodes the byte 255, whereas the latter escape sequence represents the code point 255, which is encoded as two bytes in UTF-8:

julia> b"\xff"
1-element Array{UInt8,1}:
 0xff

julia> b"\uff"
2-element Array{UInt8,1}:
 0xc3
 0xbf

In character literals, this distinction is glossed over and \xff is allowed to represent the code point 255, because characters always represent code points. In strings, however, \x escapes always represent bytes, not code points, whereas \u and \U escapes always represent code points, which are encoded in one or more bytes. For code points less than \u80, it happens that the UTF-8 encoding of each code point is just the single byte produced by the corresponding \x escape, so the distinction can safely be ignored. For the escapes \x80 through \xff as compared to \u80 through \uff, however, there is a major difference: the former escapes all encode single bytes, which – unless followed by very specific continuation bytes – do not form valid UTF-8 data, whereas the latter escapes all represent Unicode code points with two-byte encodings.

If this is all extremely confusing, try reading "The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets". It's an excellent introduction to Unicode and UTF-8, and may help alleviate some confusion regarding the matter.

Version Number Literals

Version numbers can easily be expressed with non-standard string literals of the form v"...". Version number literals create VersionNumber objects which follow the specifications of semantic versioning, and therefore are composed of major, minor and patch numeric values, followed by pre-release and build alpha-numeric annotations. For example, v"0.2.1-rc1+win64" is broken into major version 0, minor version 2, patch version 1, pre-release rc1 and build win64. When entering a version literal, everything except the major version number is optional, therefore e.g. v"0.2" is equivalent to v"0.2.0" (with empty pre-release/build annotations), v"2" is equivalent to v"2.0.0", and so on.

VersionNumber objects are mostly useful to easily and correctly compare two (or more) versions. For example, the constant VERSION holds Julia version number as a VersionNumber object, and therefore one can define some version-specific behavior using simple statements as:

if v"0.2" <= VERSION < v"0.3-"
    # do something specific to 0.2 release series
end

Note that in the above example the non-standard version number v"0.3-" is used, with a trailing -: this notation is a Julia extension of the standard, and it's used to indicate a version which is lower than any 0.3 release, including all of its pre-releases. So in the above example the code would only run with stable 0.2 versions, and exclude such versions as v"0.3.0-rc1". In order to also allow for unstable (i.e. pre-release) 0.2 versions, the lower bound check should be modified like this: v"0.2-" <= VERSION.

Another non-standard version specification extension allows one to use a trailing + to express an upper limit on build versions, e.g. VERSION > v"0.2-rc1+" can be used to mean any version above 0.2-rc1 and any of its builds: it will return false for version v"0.2-rc1+win64" and true for v"0.2-rc2".

It is good practice to use such special versions in comparisons (particularly, the trailing - should always be used on upper bounds unless there's a good reason not to), but they must not be used as the actual version number of anything, as they are invalid in the semantic versioning scheme.

Besides being used for the VERSION constant, VersionNumber objects are widely used in the Pkg module, to specify packages versions and their dependencies.

Raw String Literals

Raw strings without interpolation or unescaping can be expressed with non-standard string literals of the form raw"...". Raw string literals create ordinary String objects which contain the enclosed contents exactly as entered with no interpolation or unescaping. This is useful for strings which contain code or markup in other languages which use $ or \ as special characters. The exception is quotation marks that still must be escaped, e.g. raw"\"" is equivalent to "\"".