regexpi

Match regular expression (case insensitive)

Syntax

startIndex = regexpi(str,expression)

[startIndex,endIndex] = regexpi(str,expression)

out = regexpi(str,expression,outkey)

[out1,...,outN] = regexpi(str,expression,outkey1,...,outkeyN)

___ = regexpi(___,option1,...,optionM)

___ = regexpi(___,'forceCellOutput')

Description

startIndex = regexpi(str,expression) returns the starting index of each substring of str that matches the character patterns specified by the regular expression, without regard to letter case. If there are no matches, startIndex is an empty array.

[startIndex,endIndex] = regexpi(str,expression) returns the starting and ending indices of all matches.

out = regexpi(str,expression,outkey) returns the output specified by outkey. For example, if outkey is 'match', then regexpi returns the substrings that match the expression rather than their starting indices.

[out1,...,outN] = regexpi(str,expression,outkey1,...,outkeyN) returns the outputs specified by multiple output keywords, in the specified order. For example, if you specify 'match','tokens', then regexpi returns substrings that match the entire expression and tokens that match parts of the expression.

___ = regexpi(___,option1,...,optionM) modifies the search using the specified option flags. For example, specify 'matchcase' to perform a case-sensitive match. You can include any of the inputs and request any of the outputs from previous syntaxes.

___ = regexpi(___,'forceCellOutput') returns each output argument as a scalar cell. The cells contain the numeric arrays or substrings that are described as the outputs of the previous syntaxes. You can include any of the inputs and request any of the outputs from previous syntaxes.

Examples

Pattern Matching

Find words that start with c, end with t, and contain one or more vowels between them.

str = 'bat cat can car COAT court cut ct CAT-scan';
expression = 'c[aeiou]+t';
startIndex = regexpi(str,expression)

startIndex = 1×4

     5    17    28    35

Values in startIndex indicate the index of the first character of each word that matches the regular expression.

The regular expression 'c[aeiou]+t' specifies this pattern:

• c must be the first character.

• c must be followed by one of the characters inside the brackets, [aeiou].

• The bracketed pattern must occur one or more times, as indicated by the + operator.

• t must be the last character, with no characters between the bracketed pattern and the t.

Case-Sensitive Match

Match letter case in all or part of an expression.

By default, regexpi performs case-insensitive matching.

str = 'A character vector with UPPERCASE and lowercase text.';
expression = '\w*case';
matchStr = regexpi(str,expression,'match')

matchStr = 1x2 cell
    {'UPPERCASE'}    {'lowercase'}

Use the regexp function with the same syntax as regexpi to perform case-sensitive matching.

matchWithRegexp = regexp(str,expression,'match')

matchWithRegexp = 1x1 cell array
    {'lowercase'}

To disable case-sensitive matching for regexp, use the 'ignorecase' option.

matchWithIgnorecase = regexp(str,expression,'match','ignorecase')

matchWithIgnorecase = 1x2 cell
    {'UPPERCASE'}    {'lowercase'}

For multiple expressions, enable and disable case-insensitive matching for selected expressions using the (?i) and (?-i) search flags.

expression = {'(?-i)\w*case';...
              '(?i)\w*case'};
matchStr = regexp(str,expression,'match');
celldisp(matchStr)

 
matchStr{1}{1} =
 
lowercase
 
 
matchStr{2}{1} =
 
UPPERCASE
 
 
matchStr{2}{2} =
 
lowercase
 

Input Arguments

str — Input text

character vector | cell array of character vectors | string array

Input text, specified as a character vector, a cell array of character vectors, or a string array. Each character vector in a cell array, or each string in a string array, can be of any length and contain any characters.

If str and expression are string arrays or cell arrays, they must have the same dimensions.

Data Types: string | char | cell

expression — Regular expression

character vector | cell array of character vectors | string array

Regular expression, specified as a character vector, a cell array of character vectors, or a string array. Each expression can contain characters, metacharacters, operators, tokens, and flags that specify patterns to match in str.

The following tables describe the elements of regular expressions.

Metacharacters

Metacharacters represent letters, letter ranges, digits, and space characters. Use them to construct a generalized pattern of characters.

Metacharacter	Description	Example
.	Any single character, including white space	'..ain' matches sequences of five consecutive characters that end with 'ain'.
[c1c2c3]	Any character contained within the square brackets. The following characters are treated literally: $ | . * + ? and - when not used to indicate a range.	'[rp.]ain' matches 'rain' or 'pain' or '.ain'.
[^c1c2c3]	Any character not contained within the square brackets. The following characters are treated literally: $ | . * + ? and - when not used to indicate a range.	'[^*rp]ain' matches all four-letter sequences that end in 'ain', except 'rain' and 'pain' and '*ain'. For example, it matches 'gain', 'lain', or 'vain'.
[c1-c2]	Any character in the range of c1 through c2	'[A-G]' matches a single character in the range of A through G.
\w	Any alphabetic, numeric, or underscore character. For English character sets, \w is equivalent to [a-zA-Z_0-9]	'\w*' identifies a word comprised of any grouping of alphabetic, numeric, or underscore characters.
\W	Any character that is not alphabetic, numeric, or underscore. For English character sets, \W is equivalent to [^a-zA-Z_0-9]	'\W*' identifies a term that is not a word comprised of any grouping of alphabetic, numeric, or underscore characters.
\s	Any white-space character; equivalent to [ \f\n\r\t\v]	'\w*n\s' matches words that end with the letter n, followed by a white-space character.
\S	Any non-white-space character; equivalent to [^ \f\n\r\t\v]	'\d\S' matches a numeric digit followed by any non-white-space character.
\d	Any numeric digit; equivalent to [0-9]	'\d*' matches any number of consecutive digits.
\D	Any nondigit character; equivalent to [^0-9]	'\w*\D\>' matches words that do not end with a numeric digit.
\oN or \o{N}	Character of octal value N	'\o{40}' matches the space character, defined by octal 40.
\xN or \x{N}	Character of hexadecimal value N	'\x2C' matches the comma character, defined by hex 2C.

Character Representation

Operator	Description
\a	Alarm (beep)
\b	Backspace
\f	Form feed
\n	New line
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\char	Any character with special meaning in regular expressions that you want to match literally (for example, use \\ to match a single backslash)

Quantifiers

Quantifiers specify the number of times a pattern must occur in the matching text.

Quantifier	Number of Times Expression Occurs	Example
expr*	0 or more times consecutively.	'\w*' matches a word of any length.
expr?	0 times or 1 time.	'\w*(\.m)?' matches words that optionally end with the extension .m.
expr+	1 or more times consecutively.	'<img src="\w+\.gif">' matches an <img> HTML tag when the file name contains one or more characters.
expr{m,n}	At least m times, but no more than n times consecutively. {0,1} is equivalent to ?.	'\S{4,8}' matches between four and eight non-white-space characters.
expr{m,}	At least m times consecutively. {0,} and {1,} are equivalent to * and +, respectively.	'<a href="\w{1,}\.html">' matches an <a> HTML tag when the file name contains one or more characters.
expr{n}	Exactly n times consecutively. Equivalent to {n,n}.	'\d{4}' matches four consecutive digits.

Quantifiers can appear in three modes, described in the following table. q represents any of the quantifiers in the previous table.

Mode	Description	Example
exprq	Greedy expression: match as many characters as possible.	Given the text '<tr><td><p>text</p></td>', the expression '</?t.*>' matches all characters between <tr and /td>: '<tr><td><p>text</p></td>'
exprq?	Lazy expression: match as few characters as necessary.	Given the text'<tr><td><p>text</p></td>', the expression '</?t.*?>' ends each match at the first occurrence of the closing angle bracket (>): '<tr>' '<td>' '</td>'
exprq+	Possessive expression: match as much as possible, but do not rescan any portions of the text.	Given the text'<tr><td><p>text</p></td>', the expression '</?t.*+>' does not return any matches, because the closing angle bracket is captured using .*, and is not rescanned.

Grouping Operators

Grouping operators allow you to capture tokens, apply one operator to multiple elements, or disable backtracking in a specific group.

Grouping Operator	Description	Example
(expr)	Group elements of the expression and capture tokens.	'Joh?n\s(\w*)' captures a token that contains the last name of any person with the first name John or Jon.
(?:expr)	Group, but do not capture tokens.	'(?:[aeiou][^aeiou]){2}' matches two consecutive patterns of a vowel followed by a nonvowel, such as 'anon'. Without grouping, '[aeiou][^aeiou]{2}'matches a vowel followed by two nonvowels.
(?>expr)	Group atomically. Do not backtrack within the group to complete the match, and do not capture tokens.	'A(?>.*)Z' does not match 'AtoZ', although 'A(?:.*)Z' does. Using the atomic group, Z is captured using .* and is not rescanned.
(expr1|expr2)	Match expression expr1 or expression expr2. If there is a match with expr1, then expr2 is ignored. You can include ?: or ?> after the opening parenthesis to suppress tokens or group atomically.	'(let|tel)\w+' matches words that contain, but do not end, with let or tel.

Anchors

Anchors in the expression match the beginning or end of the input text or word.

Anchor	Matches the...	Example
^expr	Beginning of the input text.	'^M\w*' matches a word starting with M at the beginning of the text.
expr$	End of the input text.	'\w*m$' matches words ending with m at the end of the text.
\<expr	Beginning of a word.	'\<n\w*' matches any words starting with n.
expr\>	End of a word.	'\w*e\>' matches any words ending with e.

Lookaround Assertions

Lookaround assertions look for patterns that immediately precede or follow the intended match, but are not part of the match.

The pointer remains at the current location, and characters that correspond to the test expression are not captured or discarded. Therefore, lookahead assertions can match overlapping character groups.

Lookaround Assertion	Description	Example
expr(?=test)	Look ahead for characters that match test.	'\w*(?=ing)' matches terms that are followed by ing, such as 'Fly' and 'fall' in the input text 'Flying, not falling.'
expr(?!test)	Look ahead for characters that do not match test.	'i(?!ng)' matches instances of the letter i that are not followed by ng.
(?<=test)expr	Look behind for characters that match test.	'(?<=re)\w*' matches terms that follow 're', such as 'new', 'use', and 'cycle' in the input text 'renew, reuse, recycle'
(?<!test)expr	Look behind for characters that do not match test.	'(?<!\d)(\d)(?!\d)' matches single-digit numbers (digits that do not precede or follow other digits).

If you specify a lookahead assertion before an expression, the operation is equivalent to a logical AND.

Operation	Description	Example
(?=test)expr	Match both test and expr.	'(?=[a-z])[^aeiou]' matches consonants.
(?!test)expr	Match expr and do not match test.	'(?![aeiou])[a-z]' matches consonants.

Logical and Conditional Operators

Logical and conditional operators allow you to test the state of a given condition, and then use the outcome to determine which pattern, if any, to match next. These operators support logical OR, and if or if/else conditions.

Conditions can be tokens, lookaround operators, or dynamic expressions of the form (?@cmd). Dynamic expressions must return a logical or numeric value.

Conditional Operator	Description	Example
expr1|expr2	Match expression expr1 or expression expr2. If there is a match with expr1, then expr2 is ignored.	'(let|tel)\w+' matches words that start with let or tel.
(?(cond)expr)	If condition cond is true, then match expr.	'(?(?@ispc)[A-Z]:\\)' matches a drive name, such as C:\, when run on a Windows® system.
(?(cond)expr1|expr2)	If condition cond is true, then match expr1. Otherwise, match expr2.	'Mr(s?)\..*?(?(1)her|his) \w*' matches text that includes her when the text begins with Mrs, or that includes his when the text begins with Mr.

Token Operators

Tokens are portions of the matched text that you define by enclosing part of the regular expression in parentheses. You can refer to a token by its sequence in the text (an ordinal token), or assign names to tokens for easier code maintenance and readable output.

Ordinal Token Operator	Description	Example
(expr)	Capture in a token the characters that match the enclosed expression.	'Joh?n\s(\w*)' captures a token that contains the last name of any person with the first name John or Jon.
\N	Match the Nth token.	'<(\w+).*>.*</\1>' captures tokens for HTML tags, such as 'title' from the text '<title>Some text</title>'.
(?(N)expr1|expr2)	If the Nth token is found, then match expr1. Otherwise, match expr2.	'Mr(s?)\..*?(?(1)her|his) \w*' matches text that includes her when the text begins with Mrs, or that includes his when the text begins with Mr.

Named Token Operator	Description	Example
(?<name>expr)	Capture in a named token the characters that match the enclosed expression.	'(?<month>\d+)-(?<day>\d+)-(?<yr>\d+)' creates named tokens for the month, day, and year in an input date of the form mm-dd-yy.
\k<name>	Match the token referred to by name.	'<(?<tag>\w+).*>.*</\k<tag>>' captures tokens for HTML tags, such as 'title' from the text '<title>Some text</title>'.
(?(name)expr1|expr2)	If the named token is found, then match expr1. Otherwise, match expr2.	'Mr(?<sex>s?)\..*?(?(sex)her|his) \w*' matches text that includes her when the text begins with Mrs, or that includes his when the text begins with Mr.

Note

If an expression has nested parentheses, MATLAB^® captures tokens that correspond to the outermost set of parentheses. For example, given the search pattern '(and(y|rew))', MATLAB creates a token for 'andrew' but not for 'y' or 'rew'.

Dynamic Regular Expressions

Dynamic expressions allow you to execute a MATLAB command or a regular expression to determine the text to match.

The parentheses that enclose dynamic expressions do not create a capturing group.

Operator	Description	Example
(??expr)	Parse expr and include the resulting term in the match expression. When parsed, expr must correspond to a complete, valid regular expression. Dynamic expressions that use the backslash escape character (\) require two backslashes: one for the initial parsing of expr, and one for the complete match.	'^(\d+)((??\\w{$1}))' determines how many characters to match by reading a digit at the beginning of the match. The dynamic expression is enclosed in a second set of parentheses so that the resulting match is captured in a token. For instance, matching '5XXXXX' captures tokens for '5' and 'XXXXX'.
(??@cmd)	Execute the MATLAB command represented by cmd, and include the output returned by the command in the match expression.	'(.{2,}).?(??@fliplr($1))' finds palindromes that are at least four characters long, such as 'abba'.
(?@cmd)	Execute the MATLAB command represented by cmd, but discard any output the command returns. (Helpful for diagnosing regular expressions.)	'\w*?(\w)(?@disp($1))\1\w*' matches words that include double letters (such as pp), and displays intermediate results.

Within dynamic expressions, use the following operators to define replacement text.

Replacement Operator	Description
$& or $0	Portion of the input text that is currently a match
$`	Portion of the input text that precedes the current match
$'	Portion of the input text that follows the current match (use $'' to represent $')
$N	Nth token
$<name>	Named token
${cmd}	Output returned when MATLAB executes the command, cmd

Comments

Characters	Description	Example
(?#comment)	Insert a comment in the regular expression. The comment text is ignored when matching the input.	'(?# Initial digit)\<\d\w+' includes a comment, and matches words that begin with a number.

Search Flags

Search flags modify the behavior for matching expressions. An alternative to using a search flag within an expression is to pass an option input argument.

Flag	Description
(?-i)	Match letter case (default for regexp and regexprep).
(?i)	Do not match letter case (default for regexpi).
(?s)	Match dot (.) in the pattern with any character (default).
(?-s)	Match dot in the pattern with any character that is not a newline character.
(?-m)	Match the ^ and $ metacharacters at the beginning and end of text (default).
(?m)	Match the ^ and $ metacharacters at the beginning and end of a line.
(?-x)	Include space characters and comments when matching (default).
(?x)	Ignore space characters and comments when matching. Use '\ ' and '\#' to match space and # characters.

The expression that the flag modifies can appear either after the parentheses, such as

(?i)\w*

or inside the parentheses and separated from the flag with a colon (:), such as

(?i:\w*)

The latter syntax allows you to change the behavior for part of a larger expression.

Data Types: char | cell | string

outkey — Keyword that indicates which outputs to return

'start' (default) | 'end' | 'tokenExtents' | 'match' | 'tokens' | 'names' | 'split'

Keyword that indicates which outputs to return, specified as one of the following character vectors.

Output Keyword	Returns
'start' (default)	Starting indices of all matches, startIndex
'end'	Ending indices of all matches, endIndex
'tokenExtents'	Starting and ending indices of all tokens
'match'	Text of each substring that matches the pattern in expression
'tokens'	Text of each captured token in str
'names'	Name and text of each named token
'split'	Text of nonmatching substrings of str

Data Types: char | string

option — Search option

'once' | 'warnings' | 'matchcase' | 'emptymatch' | 'dotexceptnewline' | 'lineanchors' | ...

Search option, specified as a character vector. Options come in pairs: one option that corresponds to the default behavior, and one option that allows you to override the default. Specify only one option from a pair. Options can appear in any order.

Default	Override	Description
'all'	'once'	Match the expression as many times as possible (default), or only once.
'nowarnings'	'warnings'	Suppress warnings (default), or display them.
'ignorecase'	'matchcase'	Ignore letter case (default), or match case.
'noemptymatch'	'emptymatch'	Ignore zero length matches (default), or include them.
'dotall'	'dotexceptnewline'	Match dot with any character (default), or all except newline (\n).
'stringanchors'	'lineanchors'	Apply ^ and $ metacharacters to the beginning and end of a character vector (default), or to the beginning and end of a line. The newline character (\n) specifies the end of a line. The beginning of a line is specified as the first character, or any character that immediately follows a newline character.
'literalspacing'	'freespacing'	Include space characters and comments when matching (default), or ignore them. With freespacing, use '\ ' and '\#' to match space and # characters.

Data Types: char

Output Arguments

startIndex — Starting index of each match

row vector | cell array of row vectors

Starting indices of each match, returned as a row vector or cell array, as follows:

• If str and expression are both character vectors or string scalars, the output is a row vector (or, if there are no matches, an empty array).

• If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, the output is a cell array of row vectors. The output cell array has the same dimensions as the input array.

• If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions.

endIndex — Ending index of each match

row vector | cell array of row vectors

Ending index of each match, returned as a row vector or cell array, as follows:

• If str and expression are both character vectors or string scalars, the output is a row vector (or, if there are no matches, an empty array).

• If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, the output is a cell array of row vectors. The output cell array has the same dimensions as the input array.

• If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions.

out — Information about matches

numeric array | cell array | string array | structure array

Information about matches, returned as a numeric, cell, string, or structure array. The information in the output depends upon the value you specify for outkey, as follows.

Output Keyword	Output Description	Output Type and Dimensions
'start'	Starting indices of matches	For both 'start' and 'end': If str and expression are both character vectors or string scalars, the output is a row vector (or, if there are no matches, an empty array). If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, the output is a cell array of row vectors. The output cell array has the same dimensions as the input array. If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions.
'end'	Ending indices of matches
'tokenExtents'	Starting and ending indices of all tokens	By default, when returning all matches: If str and expression are both character vectors or string scalars, the output is a 1-by-n cell array, where n is the number of matches. Each cell contains an m-by-2 numeric array of indices, where m is the number of tokens in the match. If either str or expression is a cell array of character vectors or a string array, the output is a cell array with the same dimensions as the input array. Each cell contains a 1-by-n cell array, where each inner cell contains an m-by-2 numeric array. If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions. When you specify the 'once' option to return only one match, the output is either an m-by-2 numeric array or a cell array with the same dimensions as str and/or expression. If a token is expected at a particular index N, but is not found, then MATLAB returns extents for that token of [N,N-1].
'match'	Text of each substring that matches the pattern in expression	By default, when returning all matches: If str and expression are both character vectors or string scalars, the output is a 1-by-n array, where n is the number of matches. If str is a character vector, then the output is a cell array of character vectors. If str is a string scalar, then the output is a string array. If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, then the output is a cell array with the same dimensions as the argument that is an array. If str is a character vector or a cell array of character vectors, then the output is a cell array of character vectors. If str is a string array, then the output is a cell array in which each cell contains a string array. If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions. If str is a cell array of character vectors, then so is the output. If str is a string array, then the output is a cell array in which each cell contains a string array. When you specify the 'once' option to return only one match, the output is either a character vector, a string array, or a cell array with the same dimensions as str and expression.
'tokens'	Text of each captured token in str	By default, when returning all matches: If str and expression are both character vectors or string scalars, the output is a 1-by-n cell array, where n is the number of matches. Each cell contains a 1-by-m cell array of matches, where m is the number of tokens in the match. If str is a character vector, then the output is a cell array of character vectors. If str is a string array, then the output is a cell array in which each cell contains a string array. If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, then the output is a cell array with the same dimensions as the argument that is an array. Each cell contains a 1-by-n cell array, where each inner cell contains a 1-by-m array. If str is a character vector or a cell array of character vectors, then each inner cell contains a 1-by-m cell array. If str is a string array, then each inner cell contains a 1-by-m string array. If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions. If str is a cell array of character vectors, then so is the output. If str is a string array, then the output is a cell array in which the innermost cells contain string arrays. When you specify the 'once' option to return only one match, the output is a 1-by-m string array, cell array of character vectors, or a cell array that has the same dimensions as str and/or expression. If a token is expected at a particular index, but is not found, then MATLAB returns an empty value for the token, '' for character vectors, or "" for strings.
'names'	Name and text of each named token	For all matches: If str and expression are both character vectors or string scalars, the output is a 1-by-n structure array, where n is the number of matches. The structure field names correspond to the token names. If str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar. then the output is a cell array with the same dimensions as the argument that is an array. Each cell contains a 1-by-n structure array. If str and expression are both cell arrays or string arrays, they must have the same dimensions. The output is a cell array with the same dimensions.
'split'	Text of nonmatching substrings of str	For all matches: If str and expression are both character vectors or string scalars, the output is a 1-by-n array, where n is the number of nonmatches. If str is a character vector, then the output is a cell array of character vectors. If str is a string scalar, then the output is a string array. If either str or expression is a cell array of character vectors or a string array, and the other is a character vector or a string scalar, then the output is a cell array with the same dimensions as the input array. Each cell contains a 1-by-n cell array of character vectors. If str is a character vector or a cell array of character vectors, then the output is a cell array of character vectors. If str is a string array, then the output is a cell array in which each cell contains a string array. If str and expression are both cell arrays, they must have the same dimensions. The output is a cell array with the same dimensions. If str is a cell array of character vectors, then so is the output. If str is a string array, then the output is a cell array in which each cell contains a string array.

More About

Tokens

Tokens are portions of the matched text that correspond to portions of the regular expression. To create tokens, enclose part of the regular expression in parentheses.

For example, this expression finds a date of the form dd-mmm-yyyy, including tokens for the day, month, and year.

str = 'Here is a date: 01-Apr-2020';
expression = '(\d+)-(\w+)-(\d+)';

mydate = regexp(str,expression,'tokens');
mydate{:}

ans =

  1×3 cell array

    {'01'}    {'Apr'}    {'2020'}

You can associate names with tokens so that they are more easily identifiable:

str = 'Here is a date: 01-Apr-2020';
expression = '(?<day>\d+)-(?<month>\w+)-(?<year>\d+)';

mydate = regexp(str,expression,'names')

mydate = 

  struct with fields:

      day: '01'
    month: 'Apr'
     year: '2020'

For more information, see Tokens in Regular Expressions.

Extended Capabilities

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced before R2006a