QA's approach 2 Java - Regular Expressions

Regular Expressions:


abc…    Letters
123…    Digits
Literal escape \x
\d           Any Digit
\D          Any Non-digit character
.             Any Character
\.            Period
Grouping       [...]
Union          [a-e][i-u]
Intersection   [a-z&&[aeiou]]
Union             [a-e][i-u]

[abc]      Only a, b, or c

[^abc]    Not a, b, nor c
Range          a-z
[a-z]      Characters a to z
[0-9]      Numbers 0 to 9
\w          Any Alphanumeric character
\W         Any Non-alphanumeric character
{m}       m Repetitions
{m,n}    m to n Repetitions
*            Zero or more repetitions
+            One or more repetitions
?            Optional character
\s           Any Whitespace
\S          Any Non-whitespace character
^…$      Starts and ends
(…)       Capture Group
(a(bc))   Capture Sub-group
(.*)        Capture all
(ab|cd)   Matches ab or cd

String pattern=".ja."        "Ajay"
String pattern=".*Java.*"       "i am reading Java now"

.       Any character (may or may not match line terminators)   //Scanner sc=new Scanner(s).useDelimiter("(.)");

\d      A digit: [0-9]

\D      A non-digit: [^0-9]

\s      A whitespace character: [ \t\n\x0B\f\r]

\S      A non-whitespace character: [^\s]

\w      A word character: [a-zA-Z_0-9]

\W      A non-word character: [^\w] //Scanner sc=new Scanner(s).useDelimiter("[^\\w]");

Delimiter as space \\s or [ ]
//    Scanner sc=new Scanner(s).useDelimiter("\\s");
//      Scanner sc=new Scanner(s).useDelimiter("[ ]");

Delimiter as Tab space
//Scanner sc=new Scanner(s).useDelimiter("[\\t]");

Delimiter as Digit
//Scanner sc=new Scanner(s).useDelimiter("[\\d]");

Delimiter as 2 Digits followed by space  - Union
//Scanner sc=new Scanner(s).useDelimiter("[\\d][\\d][ ]");

Character classes:

[abc]              a, b, or c (simple class)

[^abc]             Any character except a, b, or c (negation)

[a-zA-Z]           a through z or A through Z, inclusive (range)

[a-d[m-p]]         a through d, or m through p: [a-dm-p] (union)

[a-z&&[def]]       d, e, or f (intersection)

[a-z&&[^bc]]       a through z, except for b and c: [ad-z] (subtraction)

[a-z&&[^m-p]]      a through z, and not m through p: [a-lq-z](subtraction)

(Java)                    Java word

Pattern

Pre-Defined character class

Quantifiers

The Java Pattern class can be used in two ways. You can use the Pattern.matches() method to quickly check if a text (String) matches a given regular expression. Or you can compile a Pattern instance using Pattern.compile() which can be used multiple times to match the regular expression against multiple texts.

Boolean matches = matcher.matches();

If the regular expression matches the whole text, then the matches() method returns true. If not, the matches() method returns false.

You cannot use the matches() method to search for multiple occurrences of a regular expression in a text. For that, you need to use find(), start() and end() methods.

The methods start() and end() will give the indexes into the text where the found match starts and ends. Actually end() returns the index of the character just after the end of the matching section. Thus, you can use the return values of start() and end() inside a String.substring() call.

Replaceall() method replaces all matches of the regular expression. The replacefirst() only replaces the first match.