Find the starting indices of substrings in a character vector.

First, create a character vector.

str = 'Find the starting indices of substrings in a character vector';

Find the substring in.

k = strfind(str,'in')

k = 1×5

     2    15    19    36    41

There are four instances in str.

Find the substring In.

k = strfind(str,'In')

k =

     []

Since strfind is case sensitive, the substring is not found. k is an empty array.

Find the blank spaces in str.

k = strfind(str,' ')

k = 1×9

     5     9    18    26    29    40    43    45    55

There are ten blank spaces in str.

Since R2020b

Create a character vector.

str = 'Find the letters.'

str = 
'Find the letters.'

Create a pattern that matches sequences of letters using the lettersPattern function.

pat = lettersPattern

pat = pattern
  Matching:

    lettersPattern

Find the index of each letter. While pat matches a sequence of letters having any length, strfind stops as soon as it finds a match and then proceeds to the next match. For example, 'Find' and 'F' are both matches for lettersPattern, since the number of letters for a match is not specified. But strfind matches 'F' first and returns its index. Then strfind matches 'i', and so on. (You can call lettersPattern with an optional argument that specifies the number of letters to match.)

k = strfind(str,pat)

k = 1×14

     1     2     3     4     6     7     8    10    11    12    13    14    15    16

To find the starts of words, call lettersPattern with boundaries. The letterBoundary function matches a boundary between letters and nonletter characters.

pat = letterBoundary + lettersPattern

pat = pattern
  Matching:

    letterBoundary + lettersPattern

k = strfind(str,pat)

k = 1×3

     1     6    10

For a list of functions that create pattern objects, see pattern.

Find the starting indices of substrings in a cell array of character vectors.

Create a cell array of character vectors.

str = {'How much wood would a woodchuck chuck';
       'if a woodchuck could chuck wood?'};

Find wood in str.

idx = strfind(str,'wood')

idx=2×1 cell array
    {1x2 double}
    {1x2 double}

Examine the output cell array to find the instances of wood.

idx{:,:}

ans = 1×2

    10    23

ans = 1×2

     6    28

The substring wood occurs at indices 10 and 23 in the first character vector and at indices 6 and 28 in the second character vector.

Find the occurrences of a substring in a character vector. Force strfind to return the indices of those occurrences in a cell array. Then display the indices.

Create a character vector and find the occurrences of the pattern ain.

str = 'The rain in Spain.';
k = strfind(str,'ain','ForceCellOutput',true)

k = 1x1 cell array
    {1x2 double}

strfind returns a scalar cell that contains a numeric array, which contains indices of occurrences of the substring ain in str. To access the numeric array within the cell, use curly braces.

k{1}

ans = 1×2

     6    15