Subject: Lookahead and lookbehind assertions
Author: Alex_Raj
In response to: Special characters in regular expression
Posted on: 08/13/2015 01:49:17 AM
a(?=b) -- positive lookahead: Is there any item matches 'a' which is followed by 'b'?
a(?!b) -- negative lookahead: Is there any item matches 'a' which is NOT followed by 'b'?
(?<=a)b -- positive lookbehind: Is there any item matches 'b' which is led by 'a'?
(?<!a)b -- negative lookbehind: Is there any item matches 'b' which is NOT led by 'a'?
(?=x) -- positive lookahead: Is there any item matches '' which is followed by 'x'?
(?!x) -- negative lookahead: Is there any item matches '' which is NOT followed by 'x'?
(?<=x) -- positive lookbehind: Is there any item matches '' which is led by 'x'?
(?<!x) -- negative lookbehind: Is there any item matches '' which is NOT led by 'x'?
-- Find the 'black' color used for 'cat'.
String regex = "black(?= cat)";
String input = "The black cat wears a black hat.";
System.out.println("Regex: " + regex);
System.out.println("Input: " + input);
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
boolean found = false;
while (matcher.find()) {
System.out.println("Found '" + matcher.group() +
"' from " + matcher.start() +
" to " + matcher.end());
found = true;
}
if(!found)
System.out.println("No match found!");
Regex: black(?= cat)
Input: The black cat wears a black hat.
Found 'black' from 4 to 9
-- Find the 'black' color used NOT for 'cat'.
String regex = "black(?! cat)";
String input = "The black cat wears a black hat.";
System.out.println("Regex: " + regex);
System.out.println("Input: " + input);
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
boolean found = false;
while (matcher.find()) {
System.out.println("Found '" + matcher.group() +
"' from " + matcher.start() +
" to " + matcher.end());
found = true;
}
if(!found)
System.out.println("No match found!");
Regex: black(?! cat)
Input: The black cat wears a black hat.
Found 'black' from 22 to 27
-- Find the 'cat' with 'black' color.
String regex = "(?<=black )cat";
String input = "The black cat and the white cat.";
System.out.println("Regex: " + regex);
System.out.println("Input: " + input);
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
boolean found = false;
while (matcher.find()) {
System.out.println("Found '" + matcher.group() +
"' from " + matcher.start() +
" to " + matcher.end());
found = true;
}
if(!found)
System.out.println("No match found!");
Regex: (?<=black )cat
Input: The black cat and the white cat.
Found 'cat' from 10 to 13
-- Find the 'cat' with NOT 'black' color.
String regex = "(?<!black )cat";
String input = "The black cat and the white cat.";
System.out.println("Regex: " + regex);
System.out.println("Input: " + input);
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
boolean found = false;
while (matcher.find()) {
System.out.println("Found '" + matcher.group() +
"' from " + matcher.start() +
" to " + matcher.end());
found = true;
}
if(!found)
System.out.println("No match found!");
Regex: (?<!black )cat
Input: The black cat and the white cat.
Found 'cat' from 28 to 31
>
> On 08/13/2015 12:35:51 AM
Alex_Raj wrote:
The following characters are metacharacters used in constructing regular expression. They must be escaped by '\' if they are used as their own literal characters.
Metacharacters
\ -- The backslash
[] -- Matches a single character that is contained within the brackets.
[^ ] -- Matches a single character that is NOT contained within the brackets.
[ && ] -- Denotes a intersection like [a-g&&[e-z]], which matches 'e', 'f', or 'g'
[ - ] -- Denotes a range like [a-z]. Otherwise, like [abc-], it a literal character.
^ -- Matches the starting position within the string.
$ -- Matches the ending position within the string.
() -- Defines a group or scope.
| -- Logical OR
* -- Matches the preceding element zero or more times. For example, 'ab*' matches 'a', 'ab',
-- 'abb', etc. '(ab)*' matches '', 'ab', 'abab', and so on. '[ab]*' matches '', 'a', 'b',
-- 'ab', 'ba', 'aabab', and so on.
? -- Matches the preceding element zero or one time. x? = (x|E)
+ -- Matches the preceding element one and more times. x+ = xx*
{m} -- Matches the preceding element exactly m times.
{m,} -- Matches the preceding element at least m times.
{m,n} -- Matches the preceding element at least m but not more than n times.
Predefined characters
. -- Any single character, for example, '.at' matches 'bat' 'cat', or 'hat'. But it is
-- literal character within [ ], for example, '[.]at' matches '.at' only.
\d -- A digit: [0-9]
\D -- A non-digit: [^0-9]
\s -- A whitespace: [ \t\n\x0B\f\r]
\S -- A non-whitespace: [^\s]
\w -- A word character: [a-zA-Z0-9_]
\W -- A non-word character: [^\w]
References:
