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<TITLE>LambdaMOO Programmer's Manual - Manipulating Strings</TITLE>
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<P><HR><P>
<H4><A NAME="SEC44" HREF="ProgrammersManual_toc.html#TOC44">Operations on Strings</A></H4>
<P>
<DL>
<DT><U>Function:</U> int <B>length</B> <I>(str <VAR>string</VAR>)</I>
<DD><A NAME="IDX33"></A>
Returns the number of characters in <VAR>string</VAR>. It is also permissible to
pass a list to <CODE>length()</CODE>; see the description in the next section.
</P>
<PRE>
length("foo") => 3
length("") => 0
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> str <B>strsub</B> <I>(str <VAR>subject</VAR>, str <VAR>what</VAR>, str <VAR>with</VAR> [, <VAR>case-matters</VAR>])</I>
<DD><A NAME="IDX34"></A>
Replaces all occurrences in <VAR>subject</VAR> of <VAR>what</VAR> with <VAR>with</VAR>,
performing string substitution. The occurrences are found from left to right
and all substitutions happen simultaneously. By default, occurrences of
<VAR>what</VAR> are searched for while ignoring the upper/lower case distinction.
If <VAR>case-matters</VAR> is provided and true, then case is treated as significant
in all comparisons.
</P>
<PRE>
strsub("%n is a fink.", "%n", "Fred") => "Fred is a fink."
strsub("foobar", "OB", "b") => "fobar"
strsub("foobar", "OB", "b", 1) => "foobar"
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> int <B>index</B> <I>(str <VAR>str1</VAR>, str <VAR>str2</VAR> [, <VAR>case-matters</VAR>])</I>
<DD><A NAME="IDX35"></A>
<DT><U>Function:</U> int <B>rindex</B> <I>(str <VAR>str1</VAR>, str <VAR>str2</VAR> [, <VAR>case-matters</VAR>])</I>
<DD><A NAME="IDX36"></A>
The function <CODE>index()</CODE> (<CODE>rindex()</CODE>) returns the index of the first
character of the first (last) occurrence of <VAR>str2</VAR> in <VAR>str1</VAR>, or zero
if <VAR>str2</VAR> does not occur in <VAR>str1</VAR> at all. By default the search for
an occurrence of <VAR>str2</VAR> is done while ignoring the upper/lower case
distinction. If <VAR>case-matters</VAR> is provided and true, then case is treated
as significant in all comparisons.
</P>
<PRE>
index("foobar", "o") => 2
rindex("foobar", "o") => 3
index("foobar", "x") => 0
index("foobar", "oba") => 3
index("Foobar", "foo", 1) => 0
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> int <B>strcmp</B> <I>(str <VAR>str1</VAR>, str <VAR>str2</VAR>)</I>
<DD><A NAME="IDX37"></A>
Performs a case-sensitive comparison of the two argument strings. If
<VAR>str1</VAR> is lexicographically less than <VAR>str2</VAR>, the
<CODE>strcmp()</CODE> returns a negative integer. If the two strings are
identical, <CODE>strcmp()</CODE> returns zero. Otherwise, <CODE>strcmp()</CODE>
returns a positive integer. The ASCII character ordering is used for the
comparison.
</DL>
</P>
<P>
<DL>
<DT><U>Function:</U> list <B>decode_binary</B> <I>(str <VAR>bin-string</VAR> [, <VAR>fully</VAR>])</I>
<DD><A NAME="IDX38"></A>
Returns a list of strings and/or integers representing the bytes in the binary
string <VAR>bin_string</VAR> in order. If <VAR>fully</VAR> is false or omitted, the list
contains an integer only for each non-printing, non-space byte; all other
characters are grouped into the longest possible contiguous substrings. If
<VAR>fully</VAR> is provided and true, the list contains only integers, one for each
byte represented in <VAR>bin_string</VAR>. Raises <CODE>E_INVARG</CODE> if
<VAR>bin_string</VAR> is not a properly-formed binary string. (See the early
section on MOO value types for a full description of binary strings.)
</P>
<PRE>
decode_binary("foo") => {"foo"}
decode_binary("~~foo") => {"~foo"}
decode_binary("foo~0D~0A") => {"foo", 13, 10}
decode_binary("foo~0Abar~0Abaz") => {"foo", 10, "bar", 10, "baz"}
decode_binary("foo~0D~0A", 1) => {102, 111, 111, 13, 10}
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> str <B>encode_binary</B> <I>(<VAR>arg</VAR>, ...)</I>
<DD><A NAME="IDX39"></A>
Each argument must be an integer between 0 and 255, a string, or a list
containing only legal arguments for this function. This function translates
each integer and string in turn into its binary string equivalent, returning the
concatenation of all these substrings into a single binary string. (See the
early section on MOO value types for a full description of binary strings.)
</P>
<PRE>
encode_binary("~foo") => "~7Efoo"
encode_binary({"foo", 10}, {"bar", 13}) => "foo~0Abar~0D"
encode_binary("foo", 10, "bar", 13) => "foo~0Abar~0D"
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> list <B>match</B> <I>(str <VAR>subject</VAR>, str <VAR>pattern</VAR> [, <VAR>case-matters</VAR>])</I>
<DD><A NAME="IDX40"></A>
<DT><U>Function:</U> list <B>rmatch</B> <I>(str <VAR>subject</VAR>, str <VAR>pattern</VAR> [, <VAR>case-matters</VAR>])</I>
<DD><A NAME="IDX41"></A>
The function <CODE>match()</CODE> (<CODE>rmatch()</CODE>) searches for the first (last)
occurrence of the regular expression <VAR>pattern</VAR> in the string <VAR>subject</VAR>.
If <VAR>pattern</VAR> is syntactically malformed, then <CODE>E_INVARG</CODE> is raised.
The process of matching can in some cases consume a great deal of memory in the
server; should this memory consumption become excessive, then the matching
process is aborted and <CODE>E_QUOTA</CODE> is raised.
</P>
<P>
If no match is found, the empty list is returned; otherwise, these functions
return a list containing information about the match (see below). By default,
the search ignores upper-/lower-case distinctions. If <VAR>case-matters</VAR> is
provided and true, then case is treated as significant in all comparisons.
</P>
<P>
The list that <CODE>match()</CODE> (<CODE>rmatch()</CODE>) returns contains the details
about the match made. The list is in the form:
</P>
<PRE>
{<VAR>start</VAR>, <VAR>end</VAR>, <VAR>replacements</VAR>, <VAR>subject</VAR>}
</PRE>
<P>
where <VAR>start</VAR> is the index in <VAR>subject</VAR> of the beginning of the match,
<VAR>end</VAR> is the index of the end of the match, <VAR>replacements</VAR> is a list
described below, and <VAR>subject</VAR> is the same string that was given as the
first argument to the <CODE>match()</CODE> or <CODE>rmatch()</CODE>.
</P>
<P>
The <VAR>replacements</VAR> list is always nine items long, each item itself being a
list of two integers, the start and end indices in <VAR>string</VAR> matched by some
parenthesized sub-pattern of <VAR>pattern</VAR>. The first item in
<VAR>replacements</VAR> carries the indices for the first parenthesized sub-pattern,
the second item carries those for the second sub-pattern, and so on. If there
are fewer than nine parenthesized sub-patterns in <VAR>pattern</VAR>, or if some
sub-pattern was not used in the match, then the corresponding item in
<VAR>replacements</VAR> is the list {0, -1}. See the discussion of <SAMP>`%)'</SAMP>,
below, for more information on parenthesized sub-patterns.
</P>
<PRE>
match("foo", "^f*o$") => {}
match("foo", "^fo*$") => {1, 3, {{0, -1}, ...}, "foo"}
match("foobar", "o*b") => {2, 4, {{0, -1}, ...}, "foobar"}
rmatch("foobar", "o*b") => {4, 4, {{0, -1}, ...}, "foobar"}
match("foobar", "f%(o*%)b")
=> {1, 4, {{2, 3}, {0, -1}, ...}, "foobar"}
</PRE>
<P>
<STRONG>Regular expression</STRONG> matching allows you to test whether a string fits into
a specific syntactic shape. You can also search a string for a substring that
fits a pattern.
</P>
<P>
A regular expression describes a set of strings. The simplest case is one that
describes a particular string; for example, the string <SAMP>`foo'</SAMP> when regarded
as a regular expression matches <SAMP>`foo'</SAMP> and nothing else. Nontrivial
regular expressions use certain special constructs so that they can match more
than one string. For example, the regular expression <SAMP>`foo%|bar'</SAMP> matches
either the string <SAMP>`foo'</SAMP> or the string <SAMP>`bar'</SAMP>; the regular expression
<SAMP>`c[ad]*r'</SAMP> matches any of the strings <SAMP>`cr'</SAMP>, <SAMP>`car'</SAMP>, <SAMP>`cdr'</SAMP>,
<SAMP>`caar'</SAMP>, <SAMP>`cadddar'</SAMP> and all other such strings with any number of
<SAMP>`a'</SAMP>'s and <SAMP>`d'</SAMP>'s.
</P>
<P>
Regular expressions have a syntax in which a few characters are special
constructs and the rest are <STRONG>ordinary</STRONG>. An ordinary character is a simple
regular expression that matches that character and nothing else. The special
characters are <SAMP>`$'</SAMP>, <SAMP>`^'</SAMP>, <SAMP>`.'</SAMP>, <SAMP>`*'</SAMP>, <SAMP>`+'</SAMP>, <SAMP>`?'</SAMP>,
<SAMP>`['</SAMP>, <SAMP>`]'</SAMP> and <SAMP>`%'</SAMP>. Any other character appearing in a regular
expression is ordinary, unless a <SAMP>`%'</SAMP> precedes it.
</P>
<P>
For example, <SAMP>`f'</SAMP> is not a special character, so it is ordinary, and
therefore <SAMP>`f'</SAMP> is a regular expression that matches the string <SAMP>`f'</SAMP> and
no other string. (It does <EM>not</EM>, for example, match the string
<SAMP>`ff'</SAMP>.) Likewise, <SAMP>`o'</SAMP> is a regular expression that matches only
<SAMP>`o'</SAMP>.
</P>
<P>
Any two regular expressions <VAR>a</VAR> and <VAR>b</VAR> can be concatenated. The
result is a regular expression which matches a string if <VAR>a</VAR> matches some
amount of the beginning of that string and <VAR>b</VAR> matches the rest of the
string.
</P>
<P>
As a simple example, we can concatenate the regular expressions <SAMP>`f'</SAMP> and
<SAMP>`o'</SAMP> to get the regular expression <SAMP>`fo'</SAMP>, which matches only the string
<SAMP>`fo'</SAMP>. Still trivial.
</P>
<P>
The following are the characters and character sequences that have special
meaning within regular expressions. Any character not mentioned here is not
special; it stands for exactly itself for the purposes of searching and
matching.
</P>
<DL COMPACT>
<DT><SAMP>`.'</SAMP>
<DD>
is a special character that matches any single character. Using concatenation,
we can make regular expressions like <SAMP>`a.b'</SAMP>, which matches any
three-character string that begins with <SAMP>`a'</SAMP> and ends with <SAMP>`b'</SAMP>.
<DT><SAMP>`*'</SAMP>
<DD>
is not a construct by itself; it is a suffix that means that the preceding
regular expression is to be repeated as many times as possible. In <SAMP>`fo*'</SAMP>,
the <SAMP>`*'</SAMP> applies to the <SAMP>`o'</SAMP>, so <SAMP>`fo*'</SAMP> matches <SAMP>`f'</SAMP> followed
by any number of <SAMP>`o'</SAMP>'s.
The case of zero <SAMP>`o'</SAMP>'s is allowed: <SAMP>`fo*'</SAMP> does match <SAMP>`f'</SAMP>.
<SAMP>`*'</SAMP> always applies to the <EM>smallest</EM> possible preceding expression.
Thus, <SAMP>`fo*'</SAMP> has a repeating <SAMP>`o'</SAMP>, not a repeating <SAMP>`fo'</SAMP>.
The matcher processes a <SAMP>`*'</SAMP> construct by matching, immediately, as many
repetitions as can be found. Then it continues with the rest of the pattern.
If that fails, it backtracks, discarding some of the matches of the <SAMP>`*'</SAMP>'d
construct in case that makes it possible to match the rest of the pattern. For
example, matching <SAMP>`c[ad]*ar'</SAMP> against the string <SAMP>`caddaar'</SAMP>, the
<SAMP>`[ad]*'</SAMP> first matches <SAMP>`addaa'</SAMP>, but this does not allow the next
<SAMP>`a'</SAMP> in the pattern to match. So the last of the matches of <SAMP>`[ad]'</SAMP> is
undone and the following <SAMP>`a'</SAMP> is tried again. Now it succeeds.
<DT><SAMP>`+'</SAMP>
<DD>
<SAMP>`+'</SAMP> is like <SAMP>`*'</SAMP> except that at least one match for the preceding
pattern is required for <SAMP>`+'</SAMP>. Thus, <SAMP>`c[ad]+r'</SAMP> does not match
<SAMP>`cr'</SAMP> but does match anything else that <SAMP>`c[ad]*r'</SAMP> would match.
<DT><SAMP>`?'</SAMP>
<DD>
<SAMP>`?'</SAMP> is like <SAMP>`*'</SAMP> except that it allows either zero or one match for
the preceding pattern. Thus, <SAMP>`c[ad]?r'</SAMP> matches <SAMP>`cr'</SAMP> or <SAMP>`car'</SAMP> or
<SAMP>`cdr'</SAMP>, and nothing else.
<DT><SAMP>`[ ... ]'</SAMP>
<DD>
<SAMP>`['</SAMP> begins a <STRONG>character set</STRONG>, which is terminated by a <SAMP>`]'</SAMP>. In
the simplest case, the characters between the two brackets form the set. Thus,
<SAMP>`[ad]'</SAMP> matches either <SAMP>`a'</SAMP> or <SAMP>`d'</SAMP>, and <SAMP>`[ad]*'</SAMP> matches any
string of <SAMP>`a'</SAMP>'s and <SAMP>`d'</SAMP>'s (including the empty string), from which it
follows that <SAMP>`c[ad]*r'</SAMP> matches <SAMP>`car'</SAMP>, etc.
Character ranges can also be included in a character set, by writing two
characters with a <SAMP>`-'</SAMP> between them. Thus, <SAMP>`[a-z]'</SAMP> matches any
lower-case letter. Ranges may be intermixed freely with individual characters,
as in <SAMP>`[a-z$%.]'</SAMP>, which matches any lower case letter or <SAMP>`$'</SAMP>,
<SAMP>`%'</SAMP> or period.
Note that the usual special characters are not special any more inside a
character set. A completely different set of special characters exists inside
character sets: <SAMP>`]'</SAMP>, <SAMP>`-'</SAMP> and <SAMP>`^'</SAMP>.
To include a <SAMP>`]'</SAMP> in a character set, you must make it the first character.
For example, <SAMP>`[]a]'</SAMP> matches <SAMP>`]'</SAMP> or <SAMP>`a'</SAMP>. To include a <SAMP>`-'</SAMP>,
you must use it in a context where it cannot possibly indicate a range: that
is, as the first character, or immediately after a range.
<DT><SAMP>`[^ ... ]'</SAMP>
<DD>
<SAMP>`[^'</SAMP> begins a <STRONG>complement character set</STRONG>, which matches any character
except the ones specified. Thus, <SAMP>`[^a-z0-9A-Z]'</SAMP> matches all characters
<EM>except</EM> letters and digits.
<SAMP>`^'</SAMP> is not special in a character set unless it is the first character.
The character following the <SAMP>`^'</SAMP> is treated as if it were first (it may be
a <SAMP>`-'</SAMP> or a <SAMP>`]'</SAMP>).
<DT><SAMP>`^'</SAMP>
<DD>
is a special character that matches the empty string -- but only if at the
beginning of the string being matched. Otherwise it fails to match anything.
Thus, <SAMP>`^foo'</SAMP> matches a <SAMP>`foo'</SAMP> which occurs at the beginning of the
string.
<DT><SAMP>`$'</SAMP>
<DD>
is similar to <SAMP>`^'</SAMP> but matches only at the <EM>end</EM> of the string. Thus,
<SAMP>`xx*$'</SAMP> matches a string of one or more <SAMP>`x'</SAMP>'s at the end of the
string.
<DT><SAMP>`%'</SAMP>
<DD>
has two functions: it quotes the above special characters (including <SAMP>`%'</SAMP>),
and it introduces additional special constructs.
Because <SAMP>`%'</SAMP> quotes special characters, <SAMP>`%$'</SAMP> is a regular expression
that matches only <SAMP>`$'</SAMP>, and <SAMP>`%['</SAMP> is a regular expression that matches
only <SAMP>`['</SAMP>, and so on.
For the most part, <SAMP>`%'</SAMP> followed by any character matches only that
character. However, there are several exceptions: characters that, when
preceded by <SAMP>`%'</SAMP>, are special constructs. Such characters are always
ordinary when encountered on their own.
No new special characters will ever be defined. All extensions to the regular
expression syntax are made by defining new two-character constructs that begin
with <SAMP>`%'</SAMP>.
<DT><SAMP>`%|'</SAMP>
<DD>
specifies an alternative. Two regular expressions <VAR>a</VAR> and <VAR>b</VAR> with
<SAMP>`%|'</SAMP> in between form an expression that matches anything that either
<VAR>a</VAR> or <VAR>b</VAR> will match.
Thus, <SAMP>`foo%|bar'</SAMP> matches either <SAMP>`foo'</SAMP> or <SAMP>`bar'</SAMP> but no other
string.
<SAMP>`%|'</SAMP> applies to the largest possible surrounding expressions. Only a
surrounding <SAMP>`%( ... %)'</SAMP> grouping can limit the grouping power of
<SAMP>`%|'</SAMP>.
Full backtracking capability exists for when multiple <SAMP>`%|'</SAMP>'s are used.
<DT><SAMP>`%( ... %)'</SAMP>
<DD>
is a grouping construct that serves three purposes:
<OL>
<LI>
To enclose a set of <SAMP>`%|'</SAMP> alternatives for other operations. Thus,
<SAMP>`%(foo%|bar%)x'</SAMP> matches either <SAMP>`foox'</SAMP> or <SAMP>`barx'</SAMP>.
<LI>
To enclose a complicated expression for a following <SAMP>`*'</SAMP>, <SAMP>`+'</SAMP>, or
<SAMP>`?'</SAMP> to operate on. Thus, <SAMP>`ba%(na%)*'</SAMP> matches <SAMP>`bananana'</SAMP>, etc.,
with any number of <SAMP>`na'</SAMP>'s, including none.
<LI>
To mark a matched substring for future reference.
</OL>
This last application is not a consequence of the idea of a parenthetical
grouping; it is a separate feature that happens to be assigned as a second
meaning to the same <SAMP>`%( ... %)'</SAMP> construct because there is no conflict
in practice between the two meanings. Here is an explanation of this feature:
<DT><SAMP>`%<VAR>digit</VAR>'</SAMP>
<DD>
After the end of a <SAMP>`%( ... %)'</SAMP> construct, the matcher remembers the
beginning and end of the text matched by that construct. Then, later on in the
regular expression, you can use <SAMP>`%'</SAMP> followed by <VAR>digit</VAR> to mean
"match the same text matched by the <VAR>digit</VAR>'th <SAMP>`%( ... %)'</SAMP>
construct in the pattern." The <SAMP>`%( ... %)'</SAMP> constructs are numbered
in the order that their <SAMP>`%('</SAMP>'s appear in the pattern.
The strings matching the first nine <SAMP>`%( ... %)'</SAMP> constructs appearing
in a regular expression are assigned numbers 1 through 9 in order of their
beginnings. <SAMP>`%1'</SAMP> through <SAMP>`%9'</SAMP> may be used to refer to the text
matched by the corresponding <SAMP>`%( ... %)'</SAMP> construct.
For example, <SAMP>`%(.*%)%1'</SAMP> matches any string that is composed of two
identical halves. The <SAMP>`%(.*%)'</SAMP> matches the first half, which may be
anything, but the <SAMP>`%1'</SAMP> that follows must match the same exact text.
<DT><SAMP>`%b'</SAMP>
<DD>
matches the empty string, but only if it is at the beginning or
end of a word. Thus, <SAMP>`%bfoo%b'</SAMP> matches any occurrence of
<SAMP>`foo'</SAMP> as a separate word. <SAMP>`%bball%(s%|%)%b'</SAMP> matches
<SAMP>`ball'</SAMP> or <SAMP>`balls'</SAMP> as a separate word.
For the purposes of this construct and the five that follow, a word is defined
to be a sequence of letters and/or digits.
<DT><SAMP>`%B'</SAMP>
<DD>
matches the empty string, provided it is <EM>not</EM> at the beginning or
end of a word.
<DT><SAMP>`%<'</SAMP>
<DD>
matches the empty string, but only if it is at the beginning
of a word.
<DT><SAMP>`%>'</SAMP>
<DD>
matches the empty string, but only if it is at the end of a word.
<DT><SAMP>`%w'</SAMP>
<DD>
matches any word-constituent character (i.e., any letter or digit).
<DT><SAMP>`%W'</SAMP>
<DD>
matches any character that is not a word constituent.
</DL>
</DL>
<P>
<DL>
<DT><U>Function:</U> str <B>substitute</B> <I>(str <VAR>template</VAR>, list <VAR>subs</VAR>)</I>
<DD><A NAME="IDX42"></A>
Performs a standard set of substitutions on the string <VAR>template</VAR>, using
the information contained in <VAR>subs</VAR>, returning the resulting, transformed
<VAR>template</VAR>. <VAR>Subs</VAR> should be a list like those returned by
<CODE>match()</CODE> or <CODE>rmatch()</CODE> when the match succeeds; otherwise,
<CODE>E_INVARG</CODE> is raised.
</P>
<P>
In <VAR>template</VAR>, the strings <SAMP>`%1'</SAMP> through <SAMP>`%9'</SAMP> will be replaced by
the text matched by the first through ninth parenthesized sub-patterns when
<CODE>match()</CODE> or <CODE>rmatch()</CODE> was called. The string <SAMP>`%0'</SAMP> in
<VAR>template</VAR> will be replaced by the text matched by the pattern as a whole
when <CODE>match()</CODE> or <CODE>rmatch()</CODE> was called. The string <SAMP>`%%'</SAMP> will
be replaced by a single <SAMP>`%'</SAMP> sign. If <SAMP>`%'</SAMP> appears in <VAR>template</VAR>
followed by any other character, <CODE>E_INVARG</CODE> will be raised.
</P>
<PRE>
subs = match("*** Welcome to LambdaMOO!!!", "%(%w*%) to %(%w*%)");
substitute("I thank you for your %1 here in %2.", subs)
=> "I thank you for your Welcome here in LambdaMOO."
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> str <B>crypt</B> <I>(str <VAR>text</VAR> [, str <VAR>salt</VAR>])</I>
<DD><A NAME="IDX43"></A>
Encrypts the given <VAR>text</VAR> using the standard UNIX encryption method. If
provided, <VAR>salt</VAR> should be a string at least two characters long, the first
two characters of which will be used as the extra encryption "salt" in the
algorithm. If <VAR>salt</VAR> is not provided, a random pair of characters is used.
In any case, the salt used is also returned as the first two characters of the
resulting encrypted string.
</P>
<P>
Aside from the possibly-random selection of the salt, the encryption algorithm
is entirely deterministic. In particular, you can test whether or not a given
string is the same as the one used to produce a given piece of encrypted text;
simply extract the first two characters of the encrypted text and pass the
candidate string and those two characters to <CODE>crypt()</CODE>. If the result is
identical to the given encrypted text, then you've got a match.
</P>
<PRE>
crypt("foobar") => "J3fSFQfgkp26w"
crypt("foobar", "J3") => "J3fSFQfgkp26w"
crypt("mumble", "J3") => "J3D0.dh.jjmWQ"
crypt("foobar", "J4") => "J4AcPxOJ4ncq2"
</PRE>
</DL>
<P>
<DL>
<DT><U>Function:</U> str <B>string_hash</B> <I>(str <VAR>text</VAR>)</I>
<DD><A NAME="IDX44"></A>
<DT><U>Function:</U> str <B>binary_hash</B> <I>(str <VAR>bin-string</VAR>)</I>
<DD><A NAME="IDX45"></A>
Returns a 32-character hexadecimal string encoding the result of applying the
MD5 cryptographically secure hash function to the contents of the string
<VAR>text</VAR> or the binary string <VAR>bin-string</VAR>. MD5, like other such
functions, has the property that, if
<PRE>
string_hash(<VAR>x</VAR>) == string_hash(<VAR>y</VAR>)
</PRE>
<P>
then, almost certainly,
<PRE>
equal(<VAR>x</VAR>, <VAR>y</VAR>)
</PRE>
<P>
This can be useful, for example, in certain networking applications: after
sending a large piece of text across a connection, also send the result of
applying <CODE>string_hash()</CODE> to the text; if the destination site also
applies <CODE>string_hash()</CODE> to the text and gets the same result, you can be
quite confident that the large text has arrived unchanged.
</DL>
</P>
<P><HR><P>
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