Question

Hello I have a question about RegEx. I am currently trying to find a way to grab a substring of any letter followed by any two numbers such as: d09.

I came up with the RegEx ^[a-z]{1}[0-9]{2}$ and ran it on the string

sedfdhajkldsfakdsakvsdfasdfr30.reed.op.1xp0

However, it never finds r30, the code below shows my approach in Java.

Pattern pattern = Pattern.compile("^[a-z]{1}[0-9]{2}$");
Matcher matcher = pattern.matcher("sedfdhajkldsfakdsakvsdfasdfr30.reed.op.1xp0");

if(matcher.matches())
    System.out.println(matcher.group(1));

it never prints out anything because matcher never finds the substring (when I run it through the debugger), what am I doing wrong?

Answer 1

There are three errors:

1. Your expression contains anchors. ^ matches only at the start of the string, and $ only matches at the end. So your regular expression will match "r30" but not "foo_r30_bar". You are searching for a substring so you should remove the anchors.

2. The matches should be find.

3. You don’t have a group 1 because you have no parentheses in your regular expression. Use group() instead of group(1).

Try this:

Pattern pattern = Pattern.compile("[a-z][0-9]{2}");
Matcher matcher = pattern.matcher("sedfdhajkldsfakdsakvsdfasdfr30.reed.op.1xp0");

if(matcher.find()) {
    System.out.println(matcher.group());    
}

ideone

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Matcher Documentation

A matcher is created from a pattern by invoking the pattern’s matcher method. Once created, a matcher can be used to perform three different kinds of match operations:

• The matches method attempts to match the entire input sequence against the pattern.
• The lookingAt method attempts to match the input sequence, starting at the beginning, against the pattern.
• The find method scans the input sequence looking for the next subsequence that matches the pattern.