/Regular Expressions/

In Java

Credits

• The Java Tutorials: Regular Expressions• docs.oracle.com/javase

/tutorial /essential/regex/

Regex

• Regular expressions are a way to describe a set of strings based on common characteristics shared by each string in the set.

• They can be used to search, edit, or manipulate text and data.

• They are created with a specific syntax.

Regex in Java

• Regex in Java is similar to Perl• The java.util.regex package primarily consists

of three classes: Pattern, Matcher, and PatternSyntaxException.

Pattern & PatternSyntaxException

• You can think of this as the regular expression wrapper object.

• You get a Pattern by calling:– Pattern.compile(“RegularExpressionString”);

• If your “RegularExpressionString” is invalid, you will get the PatternSyntaxException.

Matcher

• You can think of this as the search result object.

• You can get a matcher object by calling:– myPattern.matcher(“StringToBeSearched”);

• You use it by calling:– myMatcher.find()

• Then call any number of methods on myMatcher to see attributes of the result.

Regex Test Harness

• The tutorials give a test harness that uses the Console class. It doesn’t work in any IDE.

• So I rewrote it to use Basic I/O

CODE DEMOIt’s time for…

Regex

• Test harness output example. • Input is given in Bold.

Enter your regex: foo Enter input string to search: foofooFound ‘foo’ at index 0, ending at index 3. Found ‘foo’ at index 3, ending at index 6.

Indexing

Metacharacters

• <([{\^-=$!|]})?*+.>• Precede a metacharacter with a ‘\’ to treat it

as a ordinary character.• Or use \Q and \E to begin and end a literal

quote.

Metacharacters

Enter your regex: cat. Enter input string to search: cats Found ‘cats’ at index 0, ending at index 4.

Character ClassesConstruct Description[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, 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)

Character Class

Enter your regex: [bcr]at Enter input string to search: rat I found the text "rat" starting at index 0 and ending at index 3.

Enter input string to search: cat Found "cat" at index 0, ending at index 3.

Character Class: Negation

Enter your regex: [^bcr]at Enter input string to search: rat No match found.

Enter input string to search: hat Found "hat" at index 0, ending at index 3.

Character Class: Range

Enter your regex: foo[1-5]Enter input string to search: foo5Found "foo5" at index 0, ending at index 4.

Enter input string to search: foo6 No match found.

Character Class: Union

Enter your regex: [0-4[6-8]] Enter input string to search: 0 Found "0" at index 0, ending at index 1.

Enter input string to search: 5 No match found.

Enter input string to search: 6 Found "6" starting at index 0, ending at index 1.

Character Class: Intersection

Enter your regex: [0-9&&[345]] Enter input string to search: 5 Found "5" at index 0, ending at index 1.

Enter input string to search: 2 No match found.

Character Class: Subtraction

Enter your regex: [0-9&&[^345]]Enter input string to search: 5 No match found.

Predefined Character Classes

Construct Description

. Any character (may or may not match line terminators)

\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]

Predefined Character Classes (cont.)

• To summarize:– \d matches all digits– \s matches spaces– \w matches word characters

• Whereas a capital letter is the opposite:– \D matches non-digits– \S matches non-spaces– \W matches non-word characters

QuantifiersGreedy Reluctant Possessive MeaningX? X?? X?+ X, once or not at all

X* X*? X*+ X, zero or more times

X+ X+? X++ X, one or more times

X{n} X{n}? X{n}+ X, exactly n timesX{n,} X{n,}? X{n,}+ X, at least n times

X{n,m} X{n,m}? X{n,m}+ X, at least n but not more than m times

Ignore Greedy, Reluctant, and Possessive

For now.

Zero Length Match

• In the regexes ‘a?’ and ‘a*’ each allow for zero occurrences of the letter a.

Enter your regex: a* Enter input string to search: aaFound “aa" at index 0, ending at index 2.Found “” at index 2, ending at index 2.

Quatifiers: Exact

Enter your regex: a{3}Enter input string to search: aaNo match found.

Enter input string to search: aaaa Found "aaa" at index 0, ending at index 3.

Quantifiers: At Least, No Greater

Enter your regex: a{3,} Enter input string to search: aaaaaaaaaFound "aaaaaaaaa" at index 0, ending at index 9.

Enter your regex: a{3,6} Enter input string to search: aaaaaaaaa Found "aaaaaa" at index 0, ending at index 6. Found "aaa" at index 6, ending at index 9.

Quantifiers

• "abc+" – Means "a, followed by b, followed by (c one or

more times)".– “abcc” = match!, “abbc” = no match

• “[abc]+”– Means “(a, b, or c) one or more times)– “bba” = match!

Greedy, Reluctant, and Possessive

• Greedy– The whole input is validated, end characters are

consecutively left off as needed• Reluctant– No input is validated, beginning characters are

consecutively added as needed• Possessive– The whole input is validated, no retries are made

Greedy

Enter your regex: .*foo Enter input string to search: xfooxxxxxxfooFound "xfooxxxxxxfoo" at index 0, ending at index 13.

Reluctant

Enter your regex: .*?fooEnter input string to search: xfooxxxxxxfooFound "xfoo" at index 0, ending at index 4. Found "xxxxxxfoo" at index 4, ending at index 13.

Possessive

Enter your regex: .*+fooEnter input string to search: xfooxxxxxxfooNo match found.

Capturing Group

• Capturing groups are a way to treat multiple characters as a single unit.

• They are created by placing the characters to be grouped inside a set of parentheses.

• “(dog)” – Means a single group containing the letters "d"

"o" and "g".

Capturing Group w/ Quantifiers

• (abc)+– Means "abc" one or more times

Capturing Groups: Numbering

• ( ( A ) ( B ( C ) ) )1. ( ( A ) ( B ( C ) ) )2. ( A )3. ( B ( C ) )4. ( C )

• The index is based on the opening parentheses.

Capturing Groups: Numbering Usage

• Some Matcher methods accept a group number as a parameter:

• int start(int group)• int end (int group)• String group (int group)

Capturing Groups: Backreferences

• The section of input matching the capturing group is saved for recall via backreference.

• Specify a backreference with ‘\’ followed by the group number.

• ’(\d\d)’– Can be recalled with the expression ‘\1’.

Capturing Groups: Backreferences

Enter your regex: (\d\d)\1Enter input string to search: 1212Found "1212" at index 0, ending at index 4.

Enter input string to search: 1234No match found.

Boundary Matchers

Boundary Construct Description^ The beginning of a line$ The end of a line\b A word boundary\B A non-word boundary\A The beginning of the input\G The end of the previous match\Z The end of the input but for the final

terminator, if any\z The end of the input

Boundary Matchers

Enter your regex: ^dog$Enter input string to search: dogFound "dog" at index 0, ending at index 3.

Enter your regex: ^dog\w*Enter input string to search: dogblahblahFound "dogblahblah" at index 0, ending at index 11.

Boundary Matchers (cont.)

Enter your regex: \bdog\b Enter input string to search: The doggie plays in the yard. No match found.

Enter your regex: \Gdog Enter input string to search: dog dog Found "dog" at index 0, ending at index 3.

Pattern Class (cont.)

• There are a number of flags that can be passed to the ‘compile’ method.

• Embeddable flag expressions are Java-specific regex that duplicates these compile flags.

• Check out ‘matches’, ‘split’, and ‘quote’ methods as well.

Matcher Class (cont.)

• The Matcher class can slice input a multitude of ways:– Index methods give the position of matches– Study methods give boolean results to queries– Replacement methods let you edit input

PatternSyntaxException (cont.)

• You get a little more than just an error message from the PatternSyntaxException.

• Check out the following methods:– public String getDescription()– public int getIndex()– public String getPattern()– public String getMessage()

The End$