Programming

Logical Operators

Logical operators do not match any specific characters. They are used to specify the context for matching an accompanying regular expression.

Operator	Usage
(expr)	Group regular expressions and capture tokens.
(?:expr)	Group regular expressions, but do not capture tokens.
(?>expr)	Group atomically.
(?#expr)	Insert a comment into the expression. Comments are ignored in matching.
expr1|expr2	Match expression expr1 or expression expr2.
^expr	Match the expression only at the beginning of the string.
expr$	Match the expression only at the end of the string.
\<expr	Match the characters when they start a word.
expr\>	Match the characters when they end a word.
\<expr\>	Match an exact word.

Grouping and Capture -- (expr)

You can group elements together using either (expr) to group and capture or (?:expr) for grouping alone. For an example of the former, see Using Tokens -- Example 1. For the latter, see the "Grouping-Only" example below.

Grouping Only -- (?:expr)

Use (?:expr) to group a consonant followed by a vowel in the palindrome pstr. Specify at least two consecutive occurrences ({2,}) of this group. Return the starting and ending indices of the matched substrings:

• pstr = 'Marge lets Norah see Sharon''s telegram';
  expr = '(?:[^aeiou][aeiou]){2,}';
  
  [mat ix1 ix2] = regexp(pstr, expr, 'match', 'start', 'end')
  mat = 
      'Nora'    'haro'    'tele'
  ix1 =
      12    23    31
  ix2 =
      15    26    34
  

Remove the grouping, and the {2,} now applies only to [aeiou]. The command is entirely different now as it looks for a consonant followed by at least two consecutive vowels:

• expr = '[^aeiou][aeiou]{2,}';
  
  [mat ix1 ix2] = regexp(pstr, expr, 'match', 'start', 'end')
  mat = 
      'see'
  ix1 =
      18
  ix2 =
      20
  

Including Comments -- (?#expr)

Use (?#expr) to add a comment to this expression that matches capitalized words in pstr. Comments are ignored in the process of finding a match:

• regexp(pstr, '(?# Match words in caps)[A-Z]\w+', 'match')
  ans = 
      'Marge'    'Norah'    'Sharon'
  

Alternative Match -- expr1|expr2

Use p₁|p₂ to pick out words in the string that start with let or tel:

• regexpi(pstr, '(let|tel)\w+', 'match')
  ans =
      'lets'    'telegram'
  

Note    Be careful about using the '|' operator within square brackets in MATLAB regular expressions. Because the '|' operator has a higher precedence than '[]', MATLAB interprets the expresssion '[A|B]' not as 'A' or 'B', but as '[A' or 'B]'. The recommended way to match "the letter A or the letter B" in a MATLAB regexp expression is to use '[AB]'.

Start and End of String Match -- ^expr, expr$

Use ^expr to match words starting with the letter m or M only when it begins the string, and expr$ to match words ending with m or M only when it ends the string:

• regexpi(pstr, '^m\w*|\w*m$', 'match')
  ans =
      'Marge'    'telegram'
  

Start and End of Word Match -- \<expr, expr\>

Use \<expr to match any words starting with n or N, or ending with e or E:

• regexpi(pstr, '\<n\w*|\w*e\>', 'match')
  ans =
      'Marge'    'Norah'    'see'
  

Exact Word Match -- \<expr\>

Use \<expr\> to match a word starting with an n or N and ending with an h or H:

• regexpi(pstr, '\<n\w*h\>', 'match')
  ans = 
      'Norah'
  

Character Representation

Lookaround Operators

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