GNU Octave - Manipulating Strings

GNU Octave Manual Version 3
by John W. Eaton, David Bateman, Søren Hauberg
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GNU Octave Manual Version 3

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5.3 Manipulating Strings

Octave supports a wide range of functions for manipulating strings. Since a string is just a matrix, simple manipulations can be accomplished using standard operators. The following example shows how to replace all blank characters with underscores.

quote = ...
  "First things first, but not necessarily in that order";
quote( quote == " " ) = "_"
=> quote = 
    First_things_first,_but_not_necessarily_in_that_order

For more complex manipulations, such as searching, replacing, and general regular expressions, the following functions come with Octave.

Function File: deblank (s): Remove trailing blanks and nulls from s. If s is a matrix, deblank trims each row to the length of longest string. If s is a cell array, operate recursively on each element of the cell array.

Function File: findstr (s, t, overlap)

Return the vector of all positions in the longer of the two strings s and t where an occurrence of the shorter of the two starts. If the optional argument overlap is nonzero, the returned vector can include overlapping positions (this is the default). For example,

findstr ("ababab", "a")
=> [ 1, 3, 5 ]
findstr ("abababa", "aba", 0)
=> [ 1, 5 ]

Function File: index (s, t)

Function File: index (s, t, direction)

Return the position of the first occurrence of the string t in the string s, or 0 if no occurrence is found. For example,

index ("Teststring", "t")
=> 4

If direction is ‘"first"’, return the first element found. If direction is ‘"last"’, return the last element found. The rindex function is equivalent to index with direction set to ‘"last"’.

Caution: This function does not work for arrays of character strings.

See also find, index

Function File: idx = strfind (str, pattern)

Function File: idx = strfind (cellstr, pattern)

Search for pattern in the string str and return the starting index of every such occurrence in the vector idx. If there is no such occurrence, or if pattern is longer than str, then idx is the empty array [].

If the cell array of strings cellstr is specified instead of the string str, then idx is a cell array of vectors, as specified above.

See also findstr, strmatch, strcmp, strncmp, strcmpi, strncmpi

Function File: strmatch (s, a, "exact"): Return indices of entries of a that match the string s. The second argument a may be a string matrix or a cell array of strings. If the third argument "exact" is not given, then s only needs to match a up to the length of s. Nul characters match blanks. Results are returned as a column vector.

Function File: [tok, rem] = strtok (str, delim)

Find all characters up to but not including the first character which is in the string delim. If rem is requested, it contains the remainder of the string, starting at the first deliminator. Leading delimiters are ignored. If delim is not specified, space is assumed.

Function File: split (s, t, n)

Divides the string s into pieces separated by t, returning the result in a string array (padded with blanks to form a valid matrix). If the optional input n is supplied, split s into at most n different pieces.

For example,

split ("Test string", "t")
=> "Tes "
        " s  "
        "ring"

split ("Test string", "t", 2)
=> "Tes    "
        " string"

Function File: strrep (s, x, y)

Replaces all occurrences of the substring x of the string s with the string y. For example,

strrep ("This is a test string", "is", "&%$")
=> "Th&%$ &%$ a test string"

Function File: substr (s, offset, len)

Return the substring of s which starts at character number offset and is len characters long.

If offset is negative, extraction starts that far from the end of the string. If len is omitted, the substring extends to the end of S.

For example,

substr ("This is a test string", 6, 9)
=> "is a test"

This function is patterned after AWK. You can get the same result by s (offset : (offset + len - 1)).

Loadable Function: [s, e, te, m, t, nm] = regexp (str, pat)

Loadable Function: [...] = regexp (str, pat, opts, ...)

Regular expression string matching. Matches pat in str and returns the position and matching substrings or empty values if there are none.

The matched pattern pat can include any of the standard regex operators, including:

.

Match any character

* + ? {}

Repetition operators, representing

*: Match zero or more times
+: Match one or more times
?: Match zero or one times
{}: Match range operator, which is of the form {n} to match exactly n times, {m,} to match m or more times, {m,n} to match between m and n times.

[...] [^...]

List operators, where for example [ab]c matches ac and bc

()

Grouping operator

|

Alternation operator. Match one of a choice of regular expressions. The alternatives must be delimited by the grouping operator () above

^ $

Anchoring operator. ^ matches the start of the string str and $ the end

In addition the following escaped characters have special meaning. Note that pat should be quoted with single quotes rather than double quotes, to avoid the escape sequences being interpreted by Octave before being passed to regexp.

\b: Match a word boundary
\B: Match within a word
\w: Matches any word character
\W: Matches any non word character
\<: Matches the beginning of a word
\>: Matches the end of a word
\s: Matches any whitespace character
\S: Matches any non whitespace character
\d: Matches any digit
\D: Matches any non-digit

The outputs of regexp by default are in the order as given below

s: The start indices of each of the matching substrings
e: The end indices of each matching substring
te: The extents of each of the matched token surrounded by (...) in pat.
m: A cell array of the text of each match.
t: A cell array of the text of each token matched.
nm: A structure containing the text of each matched named token, with the name being used as the fieldname. A named token is denoted as (?<name>...)

Particular output arguments or the order of the output arguments can be selected by additional opts arguments. These are strings and the correspondence between the output arguments and the optional argument are

	'start'	`s`
	'end'	`e`
	'tokenExtents'	`te`
	'match'	`m`
	'tokens'	`t`
	'names'	`nm`

A further optional argument is 'once', that limits the number of returned matches to the first match. Additional arguments are

‘matchcase’: Make the matching case sensitive.
‘ignorecase’: Make the matching case insensitive.
‘stringanchors’: Match the anchor characters at the beginning and end of the string.
‘lineanchors’: Match the anchor characters at the beginning and end of the line.
‘dotall’: The character . matches the newline character.
‘dotexceptnewline’: The character . matches all but the newline character.
‘freespacing’: The pattern can include arbitrary whitespace and comments starting with #.
‘literalspacing’: The pattern is taken literally.

Loadable Function: [s, e, te, m, t, nm] = regexpi (str, pat)
Loadable Function: [...] = regexpi (str, pat, opts, ...): Case insensitive regular expression string matching. Matches pat in str and returns the position and matching substrings or empty values if there are none. See regexp for more details

Loadable Function: string = regexprep (string, pat, repstr, options)

Replace matches of pat in string with repstr. The replacement can contain $i, which substitutes for the i-th set of parentheses in the match string. E.g.,

regexprep("Bill Dunn",'(\w+) (\w+)','$2, $1')

returns "Dunn, Bill" options may be zero or more of

‘once’: Replace only the first occurrence of pat in the result.
‘warnings’: This option is present for compatibility but is ignored.
‘ignorecase or matchcase’: Ignore case for the pattern matching (see regexpi). Alternatively, use (?i) or (?-i) in the pattern.
‘lineanchors and stringanchors’: Whether characters ^ and $ match the beginning and ending of lines. Alternatively, use (?m) or (?-m) in the pattern.
‘dotexceptnewline and dotall’: Whether . matches newlines in the string. Alternatively, use (?s) or (?-s) in the pattern.
‘freespacing or literalspacing’: Whether whitespace and # comments can be used to make the regular expression more readable. Alternatively, use (?x) or (?-x) in the pattern.