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5.5 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: strtrim (s)

Remove leading and trailing blanks and nulls from s. If s is a matrix, strtrim trims each row to the length of longest string. If s is a cell array, operate recursively on each element of the cell array. For example:

 
strtrim ("    abc  ")
     ⇒ "abc"

strtrim ([" abc   "; "   def   "])
     ⇒ ["abc  "; "  def"]

Function File: strtrunc (s, n)

Truncate the character string s to length n. If s is a char matrix, then the number of columns is adjusted.

If s is a cell array of strings, then the operation is performed on its members and the new cell array is returned.

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]

See also: strfind, strmatch, strcmp, strncmp, strcmpi, strncmpi, find.

Function File: idx = strchr (str, chars)
Function File: idx = strchr (str, chars, n)
Function File: idx = strchr (str, chars, n, direction)

Search for the string str for occurrences of characters from the set chars. The return value, as well as the n and direction arguments behave identically as in find.

This will be faster than using regexp in most cases.

See also: find.

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, rindex.

Function File: rindex (s, t)

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

 
rindex ("Teststring", "t")
     ⇒ 6

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. Examples:

 
strfind ("abababa", "aba")
     ⇒ [1, 3, 5]

strfind ({"abababa", "bebebe", "ab"}, "aba")
     ⇒ ans =
        {
          [1,1] =

             1   3   5

          [1,2] = [](1x0)
          [1,3] = [](1x0)
        }

See also: findstr, strmatch, strcmp, strncmp, strcmpi, strncmpi, find.

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. For example:

 
strmatch ("apple", "apple juice")
     ⇒ 1

strmatch ("apple", ["apple pie"; "apple juice"; "an apple"])
     ⇒ [1; 2]

strmatch ("apple", {"apple pie"; "apple juice"; "tomato"})
     ⇒ [1; 2]

See also: strfind, findstr, strcmp, strncmp, strcmpi, strncmpi, find.

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 delimiter. Leading delimiters are ignored. If delim is not specified, space is assumed. For example:

 
strtok ("this is the life")
     ⇒ "this"

[tok, rem] = strtok ("14*27+31", "+-*/")
     ⇒
        tok = 14
        rem = *27+31

See also: index, strsplit.

Function File: [s] = strsplit (p, sep, strip_empty)

Split a single string using one or more delimiters and return a cell array of strings. Consecutive delimiters and delimiters at boundaries result in empty strings, unless strip_empty is true. The default value of strip_empty is false.

See also: strtok.

Function File: strrep (s, x, y)

Replace all occurrences of the substring x of the string s with the string y and return the result. For example,

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

See also: regexprep, strfind, findstr.

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. It should be noted that it is recommended to quote pat in 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.

See also: regexpi, regexprep.

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 ith 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.

See also: regexp, regexpi, strrep.

Function File: regexptranslate (op, s)

Translate a string for use in a regular expression. This might include either wildcard replacement or special character escaping. The behavior can be controlled by the op that can have the values

"wildcard"

The wildcard characters ., * and ? are replaced with wildcards that are appropriate for a regular expression. For example:

 
regexptranslate ("wildcard", "*.m")
     ⇒ ".*\.m"
"escape"

The characters $.?[], that have special meaning for regular expressions are escaped so that they are treated literally. For example:

 
regexptranslate ("escape", "12.5")
     ⇒ "12\.5"

See also: regexp, regexpi, regexprep.


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