4.4.2.3 Operations on Strings

Function: int length (str string)

Returns the number of characters in string. It is also permissible to pass a list to length(); see the description in the next section.

length("foo") => 3 length("") => 0

Function: str strsub (str subject, str what, str with [, case-matters])

Replaces all occurrences in subject of what with with, performing string substitution. The occurrences are found from left to right and all substitutions happen simultaneously. By default, occurrences of what are searched for while ignoring the upper/lower case distinction. If case-matters is provided and true, then case is treated as significant in all comparisons.

strsub("%n is a fink.", "%n", "Fred") => "Fred is a fink." strsub("foobar", "OB", "b") => "fobar" strsub("foobar", "OB", "b", 1) => "foobar"

Function: int index (str str1, str str2 [, case-matters])

Function: int rindex (str str1, str str2 [, case-matters])

The function index() (rindex()) returns the index of the first character of the first (last) occurrence of str2 in str1, or zero if str2 does not occur in str1 at all. By default the search for an occurrence of str2 is done while ignoring the upper/lower case distinction. If case-matters is provided and true, then case is treated as significant in all comparisons.

index("foobar", "o") => 2 rindex("foobar", "o") => 3 index("foobar", "x") => 0 index("foobar", "oba") => 3 index("Foobar", "foo", 1) => 0

Function: int strcmp (str str1, str str2)

Performs a case-sensitive comparison of the two argument strings. If str1 is lexicographically less than str2, the strcmp() returns a negative integer. If the two strings are identical, strcmp() returns zero. Otherwise, strcmp() returns a positive integer. The ASCII character ordering is used for the comparison.

Function: list decode_binary (str bin-string [, fully])

Returns a list of strings and/or integers representing the bytes in the binary string bin_string in order. If fully 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 fully is provided and true, the list contains only integers, one for each byte represented in bin_string. Raises E_INVARG if bin_string is not a properly-formed binary string. (See section fine point on binary strings, for a full description of binary strings.)

decode_binary("foo") => {"foo"} decode_binary("~~foo") => {"~foo"} decode_binary("foo~0D~0A") => {"foo", 13, 10} decode_binary("foo~0Abar~0A") => {"foo", 10, "bar", 10} decode_binary("foo~0D~0A", 1) => {102, 111, 111, 13, 10}

Function: str encode_binary (arg, ...)

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 section fine point on binary strings, for a full description of binary strings.)

encode_binary("~foo") => "~7Efoo" encode_binary({"foo", 10}, {"bar", 13}) => "foo~0Abar~0D" encode_binary("foo", 10, "bar", 13) => "foo~0Abar~0D"

Function: list match (str subject, str pattern [, case-matters])

Function: list rmatch (str subject, str pattern [, case-matters])

The function match() (rmatch()) searches for the first (last) occurrence of the regular expression pattern in the string subject. If pattern is syntactically malformed, then E_INVARG 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 E_QUOTA is raised.

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 case-matters is provided and true, then case is treated as significant in all comparisons.

The list that match() (rmatch()) returns contains the details about the match made. The list is in the form:

{start, end, replacements, subject}

where start is the index in subject of the beginning of the match, end is the index of the end of the match, replacements is a list described below, and subject is the same string that was given as the first argument to the match() or rmatch().

The replacements list is always nine items long, each item itself being a list of two integers, the start and end indices in string matched by some parenthesized sub-pattern of pattern. The first item in replacements 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 pattern, or if some sub-pattern was not used in the match, then the corresponding item in replacements is the list {0, -1}. See the discussion of `%)', below, for more information on parenthesized sub-patterns.

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"}

Regular expression 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.

A regular expression describes a set of strings. The simplest case is one that describes a particular string; for example, the string `foo' when regarded as a regular expression matches `foo' and nothing else. Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression `foo%|bar' matches either the string `foo' or the string `bar'; the regular expression `c[ad]*r' matches any of the strings `cr', `car', `cdr', `caar', `cadddar' and all other such strings with any number of `a''s and `d''s.

Regular expressions have a syntax in which a few characters are special constructs and the rest are ordinary. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are `$', `^', `.', `*', `+', `?', `[', `]' and `%'. Any other character appearing in a regular expression is ordinary, unless a `%' precedes it.

For example, `f' is not a special character, so it is ordinary, and therefore `f' is a regular expression that matches the string `f' and no other string. (It does not, for example, match the string `ff'.) Likewise, `o' is a regular expression that matches only `o'.

Any two regular expressions a and b can be concatenated. The result is a regular expression which matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.

As a simple example, we can concatenate the regular expressions `f' and `o' to get the regular expression `fo', which matches only the string `fo'. Still trivial.

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.

`.'

is a special character that matches any single character. Using concatenation, we can make regular expressions like `a.b', which matches any three-character string that begins with `a' and ends with `b'.

`*'

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 `fo*', the `*' applies to the `o', so `fo*' matches `f' followed by any number of `o''s.

The case of zero `o''s is allowed: `fo*' does match `f'.

`*' always applies to the smallest possible preceding expression. Thus, `fo*' has a repeating `o', not a repeating `fo'.

The matcher processes a `*' 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 `*''d construct in case that makes it possible to match the rest of the pattern. For example, matching `c[ad]*ar' against the string `caddaar', the `[ad]*' first matches `addaa', but this does not allow the next `a' in the pattern to match. So the last of the matches of `[ad]' is undone and the following `a' is tried again. Now it succeeds.

`+'

`+' is like `*' except that at least one match for the preceding pattern is required for `+'. Thus, `c[ad]+r' does not match `cr' but does match anything else that `c[ad]*r' would match.

`?'

`?' is like `*' except that it allows either zero or one match for the preceding pattern. Thus, `c[ad]?r' matches `cr' or `car' or `cdr', and nothing else.

`[ ... ]'

`[' begins a character set, which is terminated by a `]'. In the simplest case, the characters between the two brackets form the set. Thus, `[ad]' matches either `a' or `d', and `[ad]*' matches any string of `a''s and `d''s (including the empty string), from which it follows that `c[ad]*r' matches `car', etc.

Character ranges can also be included in a character set, by writing two characters with a `-' between them. Thus, `[a-z]' matches any lower-case letter. Ranges may be intermixed freely with individual characters, as in `[a-z$%.]', which matches any lower case letter or `$', `%' 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: `]', `-' and `^'.

To include a `]' in a character set, you must make it the first character. For example, `[]a]' matches `]' or `a'. To include a `-', 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.

`[^ ... ]'

`[^' begins a complement character set, which matches any character except the ones specified. Thus, `[^a-z0-9A-Z]' matches all characters except letters and digits.

`^' is not special in a character set unless it is the first character. The character following the `^' is treated as if it were first (it may be a `-' or a `]').

`^'

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, `^foo' matches a `foo' which occurs at the beginning of the string.

`$'

is similar to `^' but matches only at the end of the string. Thus, `xx*$' matches a string of one or more `x''s at the end of the string.

`%'

has two functions: it quotes the above special characters (including `%'), and it introduces additional special constructs.

Because `%' quotes special characters, `%$' is a regular expression that matches only `$', and `%[' is a regular expression that matches only `[', and so on.

For the most part, `%' followed by any character matches only that character. However, there are several exceptions: characters that, when preceded by `%', 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 `%'.

`%|'

specifies an alternative. Two regular expressions a and b with `%|' in between form an expression that matches anything that either a or b will match.

Thus, `foo%|bar' matches either `foo' or `bar' but no other string.

`%|' applies to the largest possible surrounding expressions. Only a surrounding `%( ... %)' grouping can limit the grouping power of `%|'.

Full backtracking capability exists for when multiple `%|''s are used.

`%( ... %)'

is a grouping construct that serves three purposes:

1. To enclose a set of `%|' alternatives for other operations. Thus, `%(foo%|bar%)x' matches either `foox' or `barx'.

2. To enclose a complicated expression for a following `*', `+', or `?' to operate on. Thus, `ba%(na%)*' matches `bananana', etc., with any number of `na''s, including none.

3. To mark a matched substring for future reference.

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 `%( ... %)' construct because there is no conflict in practice between the two meanings. Here is an explanation of this feature:

`%digit'

After the end of a `%( ... %)' 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 `%' followed by digit to mean "match the same text matched by the digit'th `%( ... %)' construct in the pattern." The `%( ... %)' constructs are numbered in the order that their `%(''s appear in the pattern.

The strings matching the first nine `%( ... %)' constructs appearing in a regular expression are assigned numbers 1 through 9 in order of their beginnings. `%1' through `%9' may be used to refer to the text matched by the corresponding `%( ... %)' construct.

For example, `%(.*%)%1' matches any string that is composed of two identical halves. The `%(.*%)' matches the first half, which may be anything, but the `%1' that follows must match the same exact text.

`%b'

matches the empty string, but only if it is at the beginning or end of a word. Thus, `%bfoo%b' matches any occurrence of `foo' as a separate word. `%bball%(s%|%)%b' matches `ball' or `balls' 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.

`%B'

matches the empty string, provided it is not at the beginning or end of a word.

`%<'

matches the empty string, but only if it is at the beginning of a word.

`%>'

matches the empty string, but only if it is at the end of a word.

`%w'

matches any word-constituent character (i.e., any letter or digit).

`%W'

matches any character that is not a word constituent.

Function: str substitute (str template, list subs)

Performs a standard set of substitutions on the string template, using the information contained in subs, returning the resulting, transformed template. Subs should be a list like those returned by match() or rmatch() when the match succeeds; otherwise, E_INVARG is raised.

In template, the strings `%1' through `%9' will be replaced by the text matched by the first through ninth parenthesized sub-patterns when match() or rmatch() was called. The string `%0' in template will be replaced by the text matched by the pattern as a whole when match() or rmatch() was called. The string `%%' will be replaced by a single `%' sign. If `%' appears in template followed by any other character, E_INVARG will be raised.

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."

Function: str crypt (str text [, str salt])

Encrypts the given text using the standard UNIX encryption method. If provided, salt 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 salt 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.

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 crypt(). If the result is identical to the given encrypted text, then you've got a match.

crypt("foobar") => "J3fSFQfgkp26w" crypt("foobar", "J3") => "J3fSFQfgkp26w" crypt("mumble", "J3") => "J3D0.dh.jjmWQ" crypt("foobar", "J4") => "J4AcPxOJ4ncq2"

As of version 1.8.3, the entire salt (of any length) is passed to the operating system's low-level crypt function. It is unlikely, however, that all operating systems will return the same string when presented with a longer salt. Therefore, identical calls to crypt() may generate different results on different platforms, and your password verification systems will fail. Use a salt longer than two characters at your own risk.

Function: str string_hash (str text)

Function: str binary_hash (str bin-string)

Returns a 32-character hexadecimal string encoding the result of applying the MD5 cryptographically secure hash function to the contents of the string text or the binary string bin-string. MD5, like other such functions, has the property that, if

string_hash(x) == string_hash(y)

then, almost certainly,

equal(x, y)

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 string_hash() to the text; if the destination site also applies string_hash() to the text and gets the same result, you can be quite confident that the large text has arrived unchanged.