Regular Expressions
09/15/10
Andrew
Butcher
abutcher@afrolegs.com
Some Examples
Simple Character Classes
Brackets denote a character class. Simply surrounding your
class with brackets will match your input once. Supplying a +
will signify that your class will be repeated one or more
times.
[a] Just a, once.
[a-z]+ Any lower case letter one or more times.
[a-zA-Z]+ Any upper or lower case letter one or more times.
[a-zA-Z0-9]+ Any upper or lowercase letters or numbers one or more times.
Matching Strings
.* Will allow you to match the and it's closing tag .]]>
The Kleene Star or Wildcard *
The wildcard * means that the class will be repeated zero or more times.
The . stands for absolutely anything, once.
[a-z]*[0-9]+ Zero or more a-z characters followed by one or more numbers.
.* Meaning anything at all zero or more times.
.at This will match hat, cat, bat, fat, etc.
Searching At the Beginning or End of Strings
A caret ^ means match this class at the beginning of my
string. The ^ must preceed your class.
A dollar sign $ means match this class at the end of my string.
The $ must follow your class.
^[A-Z].* Caret means 'begins with' so... any upper case letter followed by
anything zero or more times.
.*;$ Anything ending in a semicolon.
Logical Expressions
Placing a bar | in your expression will signify a logical OR.
([a-b]+|[0-1]+) One or more a-b's OR one or more 1-0's.
Practical Applications
Matching an IP Address
This regular expression will match an IP Address. It's not
a valid IP Address but it looks like one, at least.
([0-9]{1,3}[.]){3}([0-9]{1,3})
Curly braces signify the amount of times this class will
occur. Also note that you have to wrap special character
such as the period in braches [.] although you could also
terminate them with a back-slash like, \:
Matching a MAC Address
See if you can do this yourself, using the IP Address
example. Valid MAC Addresses include:
C0:FF:EE:BA:BE:EE
DE:AD:BE:EF:BA:BE
sed: Stream Editor
sed stands for stream editor and allows you to apply regular
expressions to files to make quick fixes.
whitespace.txt
abutcher@shell003:~$ cat whitespace.txt
<---- Notice the leading whitespace.
abutcher@shell003:~$ sed -r 's/^[ ]+//' whitespace.txt
<---- Notice the leading whitespace.
abutcher@shell003:~$ sed -ri 's/^[ ]+//' whitespace.txt
abutcher@shell003:~$ cat whitespace.txt
<---- Notice the leading whitespace.
]]>
Regular Expressions in Java
RegexTest.java
package edu.wvu.mix.abutcher;
import java.io.IOException;
import java.util.regex.Pattern;
import jline.ConsoleReader;
public class RegexTest {
public static void main(String[] args) throws IOException {
ConsoleReader reader = new ConsoleReader();
String line;
while ((line = reader.readLine("$> ")) != null) {
if (Pattern.matches(".*;$", line)) {
System.out.println("\'" + line + "\' ends in a semicolon.");
}
if (Pattern.matches("([0-9]{1,3}[.]){3}([0-9]{1,3})", line)) {
System.out.println("\'" + line + "\' looks like an IP address.");
}
if (Pattern.matches("^[A-Z].*", line) ) {
System.out.println("\'" + line + "\' begins with a capital letter.");
}
}
}
}