Regular expressions (regexes) are patterns used to match character combinations in strings. This document discusses the history and use of regexes, provides an overview of regex vocabulary including special characters, constructs, and quantifiers, and gives examples of using regexes in Java programs for tasks like validation, splitting strings, and find-and-replace operations. It concludes with examples of validating an address and parsing a properties file using regexes.

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Introducing to Regular Expressions
Author: Mikhail Khristophorov
Last update 18 July 2017

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Agenda
 What is Regular Expressions?
 History of Regular Expressions
 Vocabulary
 Examples
 Regexes and Java
 Questions and Answers

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A regular expression, regex or regexp (sometimes called a rational expression) is, in
theoretical computer science and formal language theory, a sequence of characters that
define a search pattern. Usually this pattern is then used by string searching algorithms for
"find" or "find and replace" operations on strings.
^(https?|ftp|file)://[-a-zA-Z0-9+&@#/%?=~_|!:,.;]*[-a-zA-Z0-
9+&@#/%=~_|]
What is Regular Expressions?

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History of Regular Expressions
 1956 - mathematician Stephen Cole Kleene described regular
languages using his mathematical notation called regular sets

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History of Regular Expressions
 1968 – regexps used id QED text editor. For speed, Thompson implemented regular
expression matching by just-in-time compilation (JIT) to IBM 7094 code on the Compatible
Time-Sharing System

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History of Regular Expressions
 1987 – Perl language announced

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History of Regular Expressions
 Present time - regexes are widely supported in programming languages, text processing
programs (particular lexers), advanced text editors, and some other programs

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Vocabulary
 x The character x
 The backslash character
 0n The character with octal value 0n (0 <= n <= 7)
 0nn The character with octal value 0nn (0 <= n <= 7)
 0mnn The character with octal value 0mnn (0 <= m
<= 3, 0 <= n <= 7)
 xhh The character with hexadecimal value 0xhh
 uhhhh The character with hexadecimal value
0xhhhh
 x{h...h} The character with hexadecimal value 0xh...h
(Character.MIN_CODE_POINT <= 0xh...h <=
Character.MAX_CODE_POINT)
 t The tab character ('u0009')
 n The newline (line feed) character
('u000A')
 r The carriage-return character
('u000D')
 f The form-feed character ('u000C')
 a The alert (bell) character ('u0007')
 e The escape character ('u001B')
 cx The control character
corresponding to x
Characters

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Vocabulary
 [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)
Character classes

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Vocabulary
 . Any character (may or may not match line terminators)
 d A digit: [0-9]
 D A non-digit: [^0-9]
 h A horizontal whitespace character: [ txA0u1680u180eu2000-u200au202fu205fu3000]
 H A non-horizontal whitespace character: [^h]
 s A whitespace character: [ tnx0Bfr]
 S A non-whitespace character: [^s]
 v A vertical whitespace character: [nx0Bfrx85u2028u2029]
 V A non-vertical whitespace character: [^v]
 w A word character: [a-zA-Z_0-9]
 W A non-word character: [^w]
Predefined character classes

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Vocabulary
 ^ 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

 Linebreak matcher
 R Any Unicode linebreak sequence, is equivalent to u000Du000A|[u000Au000Bu000Cu000Du0085u2028u2029]
Boundary matchers

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Vocabulary
 (?<name>X) X, as a named-capturing group
 (?:X) X, as a non-capturing group
 (?idmsuxU-idmsuxU) Nothing, but turns match flags i d m s u x U on - off
 (?idmsux-idmsux:X) X, as a non-capturing group with the given flags i d m s u x on - off
 (?=X) X, via zero-width positive lookahead
 (?!X) X, via zero-width negative lookahead
 (?<=X) X, via zero-width positive lookbehind
 (?<!X) X, via zero-width negative lookbehind
 (?>X) X, as an independent, non-capturing group
Special constructs (named-capturing and non-capturing)

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Vocabulary
 Greedy
 X?
 X*
 X+
 X{n}
 X{n
 X{n,m},}
 Meaning
 X, once or not at all
 X, zero or more times
 X, one or more times
 X, exactly n times
 X, at least n times
 X, at least n but not more than m times
Quantifiers
 Reluctant
 X??
 X*?
 X+?
 X{n}?
 X{n,}?
 X{n,m}?
 Possessive
 X?+
 X*+
 X++
 X{n}+
 X{n,}+
 X{n,m}+

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Regexes and Java
 Pattern p = Pattern.compile("a*b");
 Matcher m = p.matcher("aaaaab");
 boolean b = m.matches();
 "This 1231 is 124 a String 1243“.replaceAll(“d”, “”);
 “a b c d”.split(“s”);

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Regexes and Java
 Lets try to validate is provided string is phone:
String phoneRegex = "(+d*)?(?d{2,3})?[d-]+";
String phone = "+38(048)720-70-01";
String notPhone = "+38(048)asb720-70-01";
Pattern phonePattern = Pattern.compile(phoneRegex);
Matcher phoneMatcher = phonePattern.matcher(phone);
System.out.println("Is phone " + phone + " " + phoneMatcher.matches());
phoneMatcher = phonePattern.matcher(notPhone);
System.out.println("Is phone " + notPhone + " " + phoneMatcher.matches());
 Or we can use Pattern.matches() method:
String phoneRegex = "(+d*)?(?d{2,3})?[d-]+";
String phone = "+38(048)720-70-01";
String notPhone = "+38(048)asb720-70-01";
System.out.println("Is phone " + phone + " " + Pattern.matches(phoneRegex, phone));
System.out.println("Is phone " + notPhone + " " + Pattern.matches(phoneRegex, notPhone));
Examples Pattern-Matcher

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Regexes and Java
 Now lets split the string into words:
String string = "This string contain several words";
Pattern wordSplitPattern = Pattern.compile("s");
String[] words = wordSplitPattern.split(string);
for (String word : words)
{
System.out.println(word);
}
 Or we can use Matcher for it:
String string = "This string contain several words";
Pattern wordSplitPattern = Pattern.compile("S+");
Matcher wordSplitMatcher = wordSplitPattern.matcher(string);
while (wordSplitMatcher.find())
{
System.out.println(wordSplitMatcher.group());
}
Examples Pattern-Matcher

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Regexes and Java
 We can validate phone number only using
String:
String phoneRegex = "(+d*)?(?d{2,3})?[d-]+";
String phone = "+38(048)720-70-01";
String notPhone = "+38(048)asb720-70-01";
System.out.println("Is phone " + phone + " " + phone.matches(phoneRegex));
System.out.println("Is phone " + notPhone + " " + notPhone.matches(phoneRegex));
 Method matches in String use Pattern-Matcher
inside:
public boolean matches(String regex) {
return Pattern.matches(regex, this);
}
Examples Strings

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Regexes and Java
 We can split string using String.split():
String string = "This string contain several words";
String[] words = string.split("s");
for (String word : words)
{
System.out.println(word);
}
 String.split() use Pattern.split() inside:
public String[] split(String regex, int limit) {
* * *
return Pattern.compile(regex).split(this, limit);
}
Examples Strings

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Regexes and Java
 We can also use regexes for
replacements:
String string = "This string contain several spaces";
System.out.println(string
.replaceAll("s", "_")
.replaceAll("spaces", "underscores"));
 We can also use regexes for
replacements:
String string = "This string contain several spaces";
System.out.println(string
.replaceFirst("s", "_")
.replaceFirst("several", "three"));
Examples Strings

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Practice
 Let’s create regex which will validate address by this format:
postal code, city, region (optional), street, house number
Address format

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Practice
 Given a properties file, which contain records, written in such format:
property_name=property_value
For example, we have such data:
version=1.0
groupId=com.luxoft.regexp
artifactId=com.luxoft.regexp
Lets build parser, which can read this file.
Split properties

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THANK YOU!