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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'

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.

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.

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 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,

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.

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, which recognizes all Unicode space characters. Delphi's Trim function considers characters U+0000 (NULL) through U+0020 (SPACE) to be whitespace.

The Braille Patterns Unicode block contains , a Braille pattern with no dots raised. The Unicode standard explicitly states that it does not act as a space. The Non breaking space can also be treated as non space for trimming purposes.