Trimming (computer programming)

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In computer programming, trimming (trim) or stripping (strip) is a string manipulation in which leading and trailing whitespace is removed from a string.

For example, the string (enclosed by apostrophes)

'  this is a test  '

would be changed, after trimming, to

'this is a test'


Left or right trimming[edit]

The most popular variants of the trim function strip only the beginning or end of the string. Typically named ltrim and rtrim respectively, or in the case of Python: lstrip and rstrip. C# uses TrimStart and TrimEnd, and Common Lisp string-left-trim and string-right-trim. Pascal and Java do not have these variants built-in, although Object Pascal (Delphi) has TrimLeft and TrimRight functions.[1]

Whitespace character list parameterization[edit]

Many trim functions have an optional parameter to specify a list of characters to trim, instead of the default whitespace characters. For example, PHP and Python allow this optional parameter, while Pascal and Java do not. With Common Lisp's string-trim function, the parameter (called character-bag) is required. The C++ Boost library defines space characters according to locale, as well as offering variants with a predicate parameter (a functor) to select which characters are trimmed.

Special empty string return value[edit]

An uncommon variant of trim returns a special result if no characters remain after the trim operation. For example, Apache Jakarta's StringUtils has a function called stripToNull which returns null in place of an empty string.

Space normalization[edit]

Space normalization is a related string manipulation where in addition to removing surrounding whitespace, any sequence of whitespace characters within the string is replaced with a single space. Space normalization is performed by the function named Trim() in spreadsheet applications (including Excel, Calc, Gnumeric, and Google Docs), and by the normalize-space() function in XSLT and XPath,

In-place trimming[edit]

While most algorithms return a new (trimmed) string, some alter the original string in-place. Notably, the Boost library allows either in-place trimming or a trimmed copy to be returned.

Definition of whitespace[edit]

The characters which are considered whitespace varies between programming languages and implementations. For example, C traditionally only counts space, tab, line feed, and carriage return characters, while languages which support Unicode typically include all Unicode space characters. Some implementations also include ASCII control codes (non-printing characters) along with whitespace characters.

Java's trim method considers ASCII spaces and control codes as whitespace, contrasting with the Java isWhitespace() method,[2] which recognizes all Unicode space characters.

Delphi's Trim function considers characters U+0000 (NULL) through U+0020 (SPACE) to be whitespace.

Non-space blanks[edit]

The Braille Patterns Unicode block contains U+2800 BRAILLE PATTERN BLANK (HTML ⠀), a Braille pattern with no dots raised. The Unicode standard explicitly states that it does not act as a space.

The Non-breaking space U+00A0   NO-BREAK SPACE (HTML   ·  ,  ) can also be treated as non-space for trimming purposes.



  1. ^ "Trim". 2013-02-02. Retrieved 2013-08-24.
  2. ^ "Character (Java 2 Platform SE 5.0)". Retrieved 2013-08-24.

External links[edit]