In programming, 'trim' or 'strip' is a string manipulation function or algorithm which removes leading and trailing whitespace from a string.
For example, in Python:
' this is a test '.strip()
will return the string:
'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 Delphi (Borland's object oriented derivative of Pascal) does have 'TrimLeft' and 'TrimRight' functions[1].
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. Common Lisp's string-trim function names the parameter ''character-bag'' and required the parameter, offering no default. 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.
Rarely, there is a variant of trim which returns a special result if no characters are left after the trim operation. For example, Apache Jakarta's 'StringUtils' has a function stripToNull which returns null in place of an empty string.
Space normalization is an alternative to trim, where as well as the removal of surrounding whitespace, any sequence of whitespace characters within the string is replaced with a single space. This is done by the normalize-space() function in XSLT and XPath.
While most algorithms return a new (trimmed) string, some algorithms alter the original string in-place. Notably, the Boost library allows either in-place trimming or a trimmed copy to be returned.
Meaning of whitespace
What is considered whitespace will vary between languages and implementations. For example, C traditionally only counts space, tab, newline and return characters, while languages supporting Unicode may count all Unicode space characters. Some implementations also count all ASCII control codes (non-printing characters) as whitespace.
For example, Java's trim method considers spaces and all control codes as whitespace. However, unlike the Java isWhitespace() method, it does not consider special Unicode space characters to be whitespace.
Usage
How to trim ''string'' in languages with built-in trim functions. These examples all return a new string and do not alter the original variable.
In languages without a built-in trim function, a custom function may need to be written, or a library found.
AWK
AWK uses regular expressions to trim[3] :
ltrim(v) = gsub(/^[ ]+/, "", v)
rtrim(v) = gsub(/[ ]+$/, "", v)
trim(v) = ltrim(v); rtrim(v)
or:
function ltrim(s) { sub(/^ ★ /, "", s); return s }
function rtrim(s) { sub(/ ★ $/, "", s); return s }
function trim(s) { return rtrim(ltrim(s)); }
C/C++
There is no standard trim function in C or C++. The equivalent function has also often been called 'EatWhitespace' in non-standard C libraries.
The open source C++ library Boost has several trim variants, including a standard one: [4]
trimmed = boost::algorithm::trim_copy(''string'');
Note that with boost's function named simply trim the input sequence is modified in-place[5], and does not return a result.
The Linux kernel also includes a strip function, strstrip(), since 2.6.18-rc1, which trims the string "in place".
The open source and portable C and C++ library The Better String Library has support for trimming as well:
btrimws (b = bfromcstr (" string "));
The following C code snippet can be used to remove white spaces from a string:
int
isspace ( char c )
{
return c
' ' || c
' ' ||
c
' ' || c
' ' ;
}
char ★
lTrim ( char ★ p )
{
while ( p && isspace( ★ p) ) ++p ;
return p ;
}
char ★
rTrim ( char ★ p )
{
char ★ temp;
if (!p) return p;
temp = (char ★ )(p + strlen(p)-1);
while ( (temp>=p) && isspace( ★ temp) ) --temp;
★ (temp+1) = ' ';
return p;
}
char ★
Trim ( char ★ p )
{
return lTrim(rTrim(p));
}
Haskell
A trim algorithm in Haskell[6]
import Data.Char
trim :: String -> String
trim = f . f
where f = reverse . dropWhile isSpace
may be interpreted as follows: ''f'' drops the preceeding whitespace, and reverses the string. ''f'' is then again applied to its own output. Note that the type signature (the second line) is optional.
JavaScript
There is no built-in trim function, but it can be added to the String class [7]:
To add a trim function to all strings:
String.prototype.trim = function() {
return this.replace(/^s ★ |s ★ $/g, "")
}
This allows the same syntax as Java to be used for JavaScript.
Perl
Perl has no built-in function, and a trimming is usually achieved through regular expressions.
Example:
''$string'' =~ s/^s+//; # remove leading whitespace
''$string'' =~ s/s+$//; # remove trailing whitespace
or:
''$string'' =~ s/^s+|s+$//g ; # remove both leading and trailing whitespace
These examples modify the value of the original variable $string.
Also available for Perl is 'StripLTSpace' in String::Strip from CPAN.
There are however two functions that are commonly used to strip whitespace from the end of strings, chomp and chop:
★ chop removes the last character from a string and returns it.
★ chomp removes the trailing newline from a string if present.
Tcl
The Tclstring command has three relevant subcommands: trim, trimright and trimleft. For each of those commands, an additional argument may be specified: a string that represents a set of characters to remove -- the default is whitespace (space, tab, newline, carriage return).
Example: trimming vowels
set string onomatopoeia
set trimmed [string trim $string aeiou] ;# result is nomatop
set r_trimmed [string trimright $string aeiou] ;# result is onomatop
set l_trimmed [string trimleft $string aeiou] ;# result is nomatopoeia
XSLT
XSLT has the function normalize-space(''string'') which strips leading and trailing whitespace and also replaces any sequence of whitespace characters (including linebreaks) with a single space.
Example:
XSLT 2.0 also includes regular expressions, providing another mechanism to perform trimming.
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