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Topic: Regular expression with examples |
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Alex_Raj
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| Regular expression with examples |
A regular expression is a sequence of characters that define a pattern which is mainly used for:
finding match -- for a given string, is there any match found for the specific pattern?
validation/assertion -- for a given string, does the string fit the specific pattern?
Example: Finding match -- the regular expression '[hc]at' matches 'cat' and 'hat' in string "The cat wears a hat."
String regex = "[hc]at";
String input = "The cat wears a 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!");
output:
Regex: [hc]at
Input: The cat wears a hat.
Found 'cat' from 4 to 7
Found 'hat' from 16 to 19
Example: Validation/assertion -- the input telephone number "415-555-1234" is good for the pattern '\d{3}-\d{3}-\d{4}'.
String regex = "\\d{3}-\\d{3}-\\d{4}";
String input = "415-555-1234";
System.out.println("Regex: " + regex);
System.out.println("Input: " + input);
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
if (matcher.matches()) {
System.out.println("Match!");
} else {
System.out.println("Does not match!");
}
output:
Regex: \d{3}-\d{3}-\d{4}
Input: 415-555-1234
Match!
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Alex_Raj
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| Special characters in regular expression |
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]
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Alex_Raj
member
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| posts: |
99 |
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| from: |
San Jose, CA |
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| Lookahead and lookbehind assertions |
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'?
The simpler forms:
(?=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'?
Example: Lookahead assertion -- 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!");
The ouput:
Regex: black(?= cat)
Input: The black cat wears a black hat.
Found 'black' from 4 to 9
Example: Lookahead assertion -- 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!");
The ouput:
Regex: black(?! cat)
Input: The black cat wears a black hat.
Found 'black' from 22 to 27
Example: Lookbehind assertion -- 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!");
The ouput:
Regex: (?<=black )cat
Input: The black cat and the white cat.
Found 'cat' from 10 to 13
Example: Lookbehind assertion -- 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!");
The ouput:
Regex: (?<!black )cat
Input: The black cat and the white cat.
Found 'cat' from 28 to 31
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Alex_Raj
member
offline |
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| posts: |
99 |
| joined: |
05/16/2006 |
| from: |
San Jose, CA |
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| Lookahead assertion examples |
Example: Find empty string '' which is immediately followed by lower-case character
String regex = "(?=[a-z])";
String input = "123a567b9";
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!");
The ouput:
Regex: (?=[a-z])
Input: 123a567b9
Found '' from 3 to 3
Found '' from 7 to 7
Example: Find string which 1) starts with '' and 2) is immediately followed by lower-case character and 3) has total length of 2 any characters
String regex = "(?=[a-z]).{2}";
String input = "123a567b9";
The ouput:
Regex: (?=[a-z]).{2}
Input: 123a567b9
Found 'a5' from 3 to 5
Found 'b9' from 7 to 9
Example: Find string which 1) starts with '' and 2) is immediately followed by lower-case character and 3) has total length of 3 any characters
String regex = "(?=[a-z]).{3}";
String input = "123a567b9";
The ouput:
Regex: (?=[a-z]).{3}
Input: 123a567b9
Found 'a56' from 3 to 6
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Alex_Raj
member
offline |
| |
| posts: |
99 |
| joined: |
05/16/2006 |
| from: |
San Jose, CA |
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| Regular expression examples |
Example: Ipv4 address
String REGEX_IPv4 = "(25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[1-9])" +
"\\." +
"(25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[1-9]|0)" +
"\\." +
"(25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[1-9]|0)" +
"\\." +
"(25[0-5]|2[0-4][0-9]|[0-1][0-9]{2}|[1-9][0-9]|[0-9])";
Example: Ipv6 address
String REGEX_IPv6 = "(?i)" + "[[a-f][0-9]:/.+*%\\[\\]]+";
Example: Email address
String REGEX_EMAIL = "[a-zA-Z0-9_+.%\\-]{1,256}" +
"\\@" +
"[a-zA-Z0-9][a-zA-Z0-9\\-]{0,64}" +
"(" +
"\\.[a-zA-Z0-9][a-zA-Z0-9\\-]{0,25}" +
")+";
Example: WEB URL
String WEB_URL = "(?i)" +
"\\b(https?|ftp|file)://" +
"[a-zA-Z0-9+&@#/%?=~_|!:,.;\\-]*[a-zA-Z0-9+&@#/%=~_|\\-]";
Example: Password strength validity
The password strength criteria requires:
8+ characters length
1+ Special Character
1+ letters in Upper Case
2+ numerals (0-9)
3+ letters in Lower Case
String REGEX_PWD =
"^" + // starting
"" + // empty string
"(?=.*[\\W])" + // followed by at least 1 special char
"(?=.*[A-Z])" + // followed by at least 1 upper-case char
"(?=.*[0-9].*[0-9])" + // followed by at least 2 digits
"(?=.*[a-z].*[a-z].*[a-z])" + // followed by at least 3 lower-case chars
".{8,}" + // followed by any (.) chars with total length of at least 8
"$"; // ending
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Alex_Raj
member
offline |
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| posts: |
99 |
| joined: |
05/16/2006 |
| from: |
San Jose, CA |
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| matches() vs find() |
matches() return true -- if the whole string matches the pattern
find() return true -- if any substring matches the pattern
Exmaple
Pattern pattern = Pattern.compile("\\d+");
System.out.println("Pattern: '\\d+'");
String input = "123A45B6";
System.out.println("Inout 1: " + input);
System.out.println("--- Result #1 -------");
Matcher matcher = pattern.matcher(input);
//System.out.println("matches()? : " + matcher.matches()); // matcher.matches() will spoil the cursor
while(matcher.find()){
System.out.println("found at : " + matcher.start());
System.out.println("found end : " + matcher.end());
System.out.println("found what : " + matcher.group());
}
System.out.println("matches()? : " + matcher.matches());
System.out.println("\n=============================\n");
pattern = Pattern.compile("\\d+");
System.out.println("Pattern: '\\d+'");
input = "123456";
System.out.println("Inout 2: " + input);
System.out.println("--- Result #2 -------");
matcher = pattern.matcher(input);
//System.out.println("matches()? : " + matcher.matches()); // matcher.matches() will spoil the cursor
while(matcher.find()){
System.out.println("found at : " + matcher.start());
System.out.println("found end : " + matcher.end());
System.out.println("found what : " + matcher.group());
}
System.out.println("matches()? : " + matcher.matches());
Output:
Pattern: '\d+'
Inout 1: 123A45B6
--- Result #1 -------
found at : 0
found end : 3
found what : 123
found at : 4
found end : 6
found what : 45
found at : 7
found end : 8
found what : 6
matches()? : false
=============================
Pattern: '\d+'
Inout 2: 123456
--- Result #2 -------
found at : 0
found end : 6
found what : 123456
matches()? : true
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posted on: 08/07/2015 12:55:27 AM
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