In computing, a delimiter is a character or a sequence of characters for specifying the boundary between separate, independent regions in data such as a text file or data stream.
In mathematics, delimiters are often used to specify the scope of an operation in an expression, and can occur both as isolated symbols (e.g., colon in "") and as a pair of opposing-looking symbols (e.g., angled brackets in ).
Delimiters aren't the only way to delineate data boundaries. For example, declarative notation indicates the length of a field at the start of the field instead of relying on delimiters. describing the method in Hollerith notation under the Fortran programming language.
Examples
Delimiters are used for a wide range of purposes. The following examples demonstrate a small fraction of their applicability.
Tabular data Tabular data, organized as rows and columns, is often delimited. A field delimiter separates the columns of a row, with each column corresponding to a field in that row, and a record delimiter separates the rows, with each row corresponding to a record. p. 141 The commonly used comma-separated values (CSV) format uses a comma to delimit fields, and an newline to delimit records. The following CSV data represents three records each with four fields. The first line is metadata that names the fields.
fname,lname,age,salary nancy,davolio,33,$30000 erin,borakova,28,$25250 tony,raphael,35,$28700
The related alternative tab-separated values (TSV) format uses the tab character instead of the comma to delimit fields. CSV and TSV formats are specific examples of a delimiter-separated values (DSV) format. A file saved in a DSV format can be classified as a flat-file database.
Bracket delimiters Bracket delimiters, also called block delimiters, region delimiters, or balanced delimiters, mark the start and end of a region of text. p. 319 Commonly used bracket delimiters include: Delimiters Description ( ) Parentheses; Lisp code is cited as recognizable by its use of parenthesesp. 3 { } Braces; also called curly brackets references C-style programming languages prominently featuring curly brackets and semicolons. [ ] Square brackets; commonly used to denote a subscript < > Angle bracketsDescribes syntax and delimiters used in HTML. " " Double quote; commonly used to denote a string literalDescribes string literals. ' ' Single quote; commonly used to denote a string literal or character literal <? ?> Used in XML to denote a processing instruction Describes XML processing instruction. p. 21. /* */ Used in many programming languages to denote a comment Describes single-line and multi-line comments. p. 72. <!-- --> Used in SGML, HTML, and XML to denote a comment <% %> Used in some web templates to specify a language boundary
ASCII information separator characters The ASCII character set designates the following 4 non-textual control characters in the C0 code range from 28 to 31 as "Information Separators", which may be used in a hierarchical or in a general (non-hierarchical) manner:
ISO 646:1983 general name, intended for application-specific use (may be non-hierarchical)ASCII 1986 name, intended for hierarchical use ASCII & codeUnicode "Control Picture"dec.hex.codeglyphINFORMATION SEPARATOR FOURIS4FILE SEPARATORFSEnd of file or between a concatenation of files 280x1CU+241C INFORMATION SEPARATOR THREEIS3GROUP SEPARATORGSBetween sections of data; not needed in simple data files 290x1DU+241D INFORMATION SEPARATOR TWOIS2RECORD SEPARATORRSEnd of a record or row 300x1EU+241E INFORMATION SEPARATOR ONEIS1UNIT SEPARATORUSBetween fields of a record, or members of a row 310x1FU+241F
Formats using ASCII information separators 1969 Internet Engineering Task Force (IETF) Request for Comments (RFC) #20 "ASCII format for Network Interchange" suggests 7-bit ASCII and uses these 4 Information Separators hierarchically.
1986 National Institute of Standards and Technology (NIST) "Fingerprint Identification - Data Format for Information Interchange" also hierarchically delineates with these 4 separators.
2007 ISO barcode standard GS1-128 uses either the special FNC1 (Function Code 1) flag or the ASCII GROUP SEPARATOR character after a variable length element.
2015 RFC #7464 "JavaScript Object Notation (JSON) Text Sequences" encodes multiple JSON texts in , each prefixed by the ASCII RECORD SEPARATOR and ending with an ASCII LINE FEED character.
2024 IETF draft format "Unicode Separated Values (USV)" allows substituting ASCII separator codes with their corresponding Unicode Control Picture (␜, ␝, ␞, ␟) to facilitate freeform Markdown-like text editing "in any typical modern editor, font, terminal, shell, search" using visible copy-pastable characters, an advantage over using ASCII's separators which aren't easily inputted and viewable in plain text editors. This substitution makes USV both machine- and human-readable and editable.
Delimiter collision Delimiter collision describes a limitation of using delimiters. When content information contains a delimiter, then the processing of the data will fail since the embedded delimiter will be incorrectly interpreted as a data boundary unless provisions are made to prevent the collision. describing solutions for embedded-delimiter problems p. 472. In XML, for example, collision can occur when content contains an angle bracket (< or >).
Each delimiter in a format can result in collision. In CSV, for example, field collision can occur when a field contains a comma (e.g., salary = "$30,000"), and record delimiter collision can occur when a field contains a newline. Both record and field delimiter collision occur frequently in CSV data.
A malicious user may seek to exploit collision. Consequently, delimiter collision can be the source of security vulnerability and exploit. Well-known examples include SQL injection and cross-site scripting in the context of SQL and HTML, respectively.
Solutions Multiple methods for avoiding collision have been devised.
Prohibit use of specified delimiters in content
One solution to avoid the risk of delimiter collision is to prohibit content from containing a specified set of designated delimiters. A downside is arbitrary binary data can't be communicated without some workaround to handle data that contains those delimiter codes.
Obfuscation Using a delimiter that is unlikely to appear in the content is an ad hoc approach that leads to limited success. It requires knowledge of expected content, guessing what won't appear in the content, and offers little security against malicious collisions.
Escape sequence A commonly used method for avoiding delimiter collision is to use an escape character or a multi-character escape sequence. A designated character (e.g. the backslash ), or sequence of characters before a character that otherwise would indicate a boundary indicates that the delimiter character is not to be treated as a boundary.
Although effective, this technique has drawbacks including:
Content can be hard to read when it contains numerous escape sequences, a problem referred to as leaning toothpick syndrome (due to use of \ to escape / in Perl regular expressions, leading to sequences such as "\/\/"); Data becomes difficult to parse via regular expression Requires a way to escape the escape sequence (a way to use the escape sequence as content) An escape sequence can be cryptic to those unfamiliar with the syntax The method does not protect against injection attacks
Higher level encoding Some systems allow any character to be represented as a sequence of characters. This allows text that otherwise is a delimiter to be encoded in the content indirectly and thus prevent delimiter collision. A drawback of this method is that character codes are relatively hard to read, understand and memorize.
For example, Perl allows a character to be encoded as the sequence where ## is the numeric value of the character code. The following shows how the sequence for double-quote () can be used to prevent collision with the delimiter that marks the begin and end of a string literal.
print "Nancy said \x22Hello World!\x22 to the crowd.";
produces the same output as:
print "Nancy said \"Hello World!\" to the crowd."; ### use escape char
Dual quoting delimiters In contrast to escape sequences and escape characters, dual delimiters provide yet another way to avoid delimiter collision. Some languages, for example, allow the use of either a single quote or a double quote to specify a string literal. For example, in Perl:
print 'Nancy said "Hello World!" to the crowd.';
produces the desired output without requiring escapes. This approach, however, only works when the string does not contain both types of quotation marks.
Doubling the delimiter Visual Basic uses a single double quote as a delimiter for the start and end of a string. To avoid collision when having a double quote inside of a string, two consecutive double-quotes are used, so:
print "Nancy said ""Hello World!"" to the crowd."
produces the desired output. Like regular escaping it can, however, become confusing when many quotes are used. For instance, the code to print the above source code is:
print "print ""Nancy said """"Hello World!"""" to the crowd."""
Configurable alternative quoting delimiters In contrast to dual delimiters, multiple delimiters are even more flexible for avoiding delimiter collision.
For example, in Perl: print qq^Nancy doesn't want to say "Hello World!" anymore.^; print qq@Nancy doesn't want to say "Hello World!" anymore.@; print qq(Nancy doesn't want to say "Hello World!" anymore.); all produce the desired output through use of quote operators, which allow any convenient character to act as a delimiter. Although this method is more flexible, few languages support it. Perl and Ruby are two that do. In Ruby, these are indicated as general delimited strings. p. 11
Content boundary A content boundary is a special type of delimiter that is specifically designed to resist delimiter collision. It works by allowing the author to specify a sequence of characters that is guaranteed to always indicate a boundary between parts in a multi-part message, with no other possible interpretation. p. 26
The delimiter is frequently generated from a random sequence of characters that is statistically improbable to occur in the content. This may be followed by an identifying mark such as a UUID, a timestamp, or some other distinguishing mark. Alternatively, the content may be scanned to guarantee that a delimiter does not appear in the text. This may allow the delimiter to be shorter or simpler, and increase the human readability of the document. (See e.g., MIME, Here documents).
Whitespace or indentation Some programming and computer languages allow the use of whitespace delimiters or indentation as a means of specifying boundaries between independent regions in text. Describes whitespace delimiters. p. 258.
Regular expression syntax
In specifying a regular expression, alternate delimiters may also be used to simplify the syntax for match and substitution operations in Perl. page 472.
For example, a simple match operation may be specified in Perl with the following syntax:
$string1 = 'Nancy said "Hello World!" to the crowd.'; # specify a target string print $string1 =~ m/[aeiou]+/; # match one or more vowels
The syntax is flexible enough to specify match operations with alternate delimiters, making it easy to avoid delimiter collision:
$string1 = 'Nancy said "http://Hello/World.htm" is not a valid address.'; # target string
print $string1 =~ m@http://@; # match using alternate regular expression delimiter print $string1 =~ m{http://}; # same as previous, but different delimiter print $string1 =~ m!http://!; # same as previous, but different delimiter.
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