Develop Reference

Regex for extracting all heading digits from a string

I am trying to extract all heading digits from a string using Java regex without writing additional code and I could not find something to work:
"12345XYZ6789ABC" should give me "12345".
"X12345XYZ6789ABC" should give me nothing
public final class NumberExtractor {
private static final Pattern DIGITS = Pattern.compile("what should be my regex here?");
public static Optional<Long> headNumber(String token) {
var matcher = DIGITS.matcher(token);
return matcher.find() ? Optional.of(Long.valueOf(matcher.group())) : Optional.empty();
}
}

Use a word boundary \b:
\b\d+
See live demo.
If you strictly want to match only digits at the start of the input, and not from each word (same thing when the input contains only one word), use ^:
^\d+
Pattern DIGITS = Pattern.compile("\\b\\d+"); // leading digits of all words
Pattern DIGITS = Pattern.compile("^\\d+"); // leading digits of input

I'd think something like "^[0-9]*" would work. There's a \d that matches other Unicode digits if you want to include them as well.
Edit: removed errant . from the string.

What is the Regex for decimal numbers in Java?

I am not quite sure of what is the correct regex for the period in Java. Here are some of my attempts. Sadly, they all meant any character.
String regex = "[0-9]*[.]?[0-9]*";
String regex = "[0-9]*['.']?[0-9]*";
String regex = "[0-9]*["."]?[0-9]*";
String regex = "[0-9]*[\.]?[0-9]*";
String regex = "[0-9]*[\\.]?[0-9]*";
String regex = "[0-9]*.?[0-9]*";
String regex = "[0-9]*\.?[0-9]*";
String regex = "[0-9]*\\.?[0-9]*";
But what I want is the actual "." character itself. Anyone have an idea?
What I'm trying to do actually is to write out the regex for a non-negative real number (decimals allowed). So the possibilities are: 12.2, 3.7, 2., 0.3, .89, 19
String regex = "[0-9]*['.']?[0-9]*";
Pattern pattern = Pattern.compile(regex);
String x = "5p4";
Matcher matcher = pattern.matcher(x);
System.out.println(matcher.find());
The last line is supposed to print false but prints true anyway. I think my regex is wrong though.

Update
To match non negative decimal number you need this regex:
^\d*\.\d+|\d+\.\d*$
or in java syntax : "^\\d*\\.\\d+|\\d+\\.\\d*$"
String regex = "^\\d*\\.\\d+|\\d+\\.\\d*$"
String string = "123.43253";
if(string.matches(regex))
System.out.println("true");
else
System.out.println("false");
Explanation for your original regex attempts:
[0-9]*\.?[0-9]*
with java escape it becomes :
"[0-9]*\\.?[0-9]*";
if you need to make the dot as mandatory you remove the ? mark:
[0-9]*\.[0-9]*
but this will accept just a dot without any number as well... So, if you want the validation to consider number as mandatory you use + ( which means one or more) instead of *(which means zero or more). That case it becomes:
[0-9]+\.[0-9]+

If you on Kotlin, use ktx:
fun String.findDecimalDigits() =
Pattern.compile("^[0-9]*\\.?[0-9]*").matcher(this).run { if (find()) group() else "" }!!

Your initial understanding was probably right, but you were being thrown because when using matcher.find(), your regex will find the first valid match within the string, and all of your examples would match a zero-length string.
I would suggest "^([0-9]+\\.?[0-9]*|\\.[0-9]+)$"

There are actually 2 ways to match a literal .. One is using backslash-escaping like you do there \\., and the other way is to enclose it inside a character class or the square brackets like [.]. Most of the special characters become literal characters inside the square brackets including .. So use \\. shows your intention clearer than [.] if all you want is to match a literal dot .. Use [] if you need to match multiple things which represents match this or that for example this regex [\\d.] means match a single digit or a literal dot

I have tested all the cases.
public static boolean isDecimal(String input) {
return Pattern.matches("^[-+]?\\d*[.]?\\d+|^[-+]?\\d+[.]?\\d*", input);
}

Java regex : Remove (double) negative look ahead and look behind

I have the following regex that matches a string to pattern:
(?i)(?<![^\\s\\p{Punct}]) : Look behind
(?![^\\s\\p{Punct}]) : Look ahead
Below is an example that demonstrates how I am using it:
public static void main(String[] args) {
String patternStart = "(?i)(?<![^\\s\\p{Punct}])", patternEnd = "(?![^\\s\\p{Punct}])";
String text = "this is some paragraph";
System.out.println(Pattern.compile(patternStart + Pattern.quote("some paragraph") + patternEnd).matcher(text).find());
}
It returns true which is expected result. However, as the regex uses double negative (i.e. negative look ahead/behind and ^), I thought removing both of the negatives should return the same result. So, I tried with the below:
String patternStart = "(?i)(?<=[\\s\\p{Punct}])", patternEnd = "(?=[\\s\\p{Punct}])";
However, it doesn't seem to be working as expected. I even tried adding ^ and/or $ in the end (of the square bracket) to match beginning/end of string, still, no luck.
Is it possible to convert these regexes into positive look-ups?

Yes, it is possible, but it is less efficient than what you have because in the positive lookarounds you need to use alternation:
String patternStart = "(?i)(?<=^|[\\s\\p{Punct}])", patternEnd = "(?=[\\s\\p{Punct}]|$)";
^^ ^^
The (?<=^|[\\s\\p{Punct}]) lookbehind requires the presence of either start of string (^) or | a whitespace or punctuation symbol ([\\s\\p{Punct}]). The positive lookahead (?=[\\s\\p{Punct}]|$) requires either a whitespace or punctuation, or the end of string.
If you just add ^ or $ into the character classes like [\\s\\p{Punct}^] and [\\s\\p{Punct}$], they will be parsed as literal caret and dollar symbols.

Regex matching word that is in the middle of any character except a letter

I'd like to know how to detect word that is between any characters except a letter from alphabet. I need this, because I'm working on a custom import organizer for Java. This is what I have already tried:
The regex expression:
[^(a-zA-Z)]InitializationEvent[^(a-zA-Z)]
I'm searching for the word "InitializationEvent".
The code snippet I've been testing on:
public void load(InitializationEvent event) {
It looks like adding space before the word helps... is the parenthesis inside of alphabet range?
I tested this in my program and it didn't work. Also I checked it on regexr.com, showing same results - class name not recognized.
Am I doing something wrong? I'm new to regex, so it might be a really basic mistake, or not. Let me know!

Lose the parentheses:
[^a-zA-Z]InitializationEvent[^a-zA-Z]
Inside [], parentheses are taken literally, and by inverting the group (^) you prevent it from matching because a ( is preceding InitializationEvent in your string.
Note, however, that the above regex will only match if InitializationEvent is neither at the beginning nor at the end of the tested string. To allow that, you can use:
(^|[^a-zA-Z])InitializationEvent([^a-zA-Z]|$)
Or, without creating any matching groups (which is supposed to be cleaner, and perform better):
(?:^|[^a-zA-Z])InitializationEvent(?:[^a-zA-Z]|$)

how to detect word that is between any characters except a letter from alphabet
This is the case where lookarounds come handy. You can use:
(?<![a-zA-Z])InitializationEvent(?![a-zA-Z])
(?<![a-zA-Z]) is negative lookbehind to assert that there is no alphabet at previous position
(?![a-zA-Z]) is negative lookahead to assert that there is no alphabet at next position
RegEx Demo

The parentheses are causing the problem, just skip them:
"[^a-zA-Z]InitializationEvent[^a-zA-Z]"
or use the predefined non-word character class which is slightly different because it also excludes numbers and the underscore:
"\\WInitializationEvent\\W"
But as it seems you want to match a class name, this might be ok because the remaining character are exactly those that are allowed in a class name.

I'm not sure about your application but from a regexp perspective you can use negative lookaheads and negative lookbehinds to define what cannot surround the String to specify a match.
I have added the negative lookahead (?![a-zA-Z]) and the negative lookbehind (?<![a-zA-Z]) in place of your [^(a-zA-Z)] originally supplied to create: (?<![a-zA-Z])InitializationEvent(?![a-zA-Z])
Quick Fiddle I created:
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class HelloWorld{
public static void main(String []args){
String pattern = "(?<![a-zA-Z])InitializationEvent(?![a-zA-Z])";
String sourceString = "public void load(InitializationEvent event) {";
String sourceString2 = "public void load(BInitializationEventA event) {";
Pattern r = Pattern.compile(pattern);
Matcher m = r.matcher(sourceString);
if (m.find( )) {
System.out.println("Found value of pattern in sourceString: " + m.group(0) );
} else {
System.out.println("NO MATCH in sourceString");
}
Matcher m2 = r.matcher(sourceString2);
if (m2.find( )) {
System.out.println("Found value of pattern in sourceString2: " + m2.group(0) );
} else {
System.out.println("NO MATCH in sourceString2");
}
}
}
output:
sh-4.3$ java -Xmx128M -Xms16M HelloWorld
Found value of pattern in sourceString: InitializationEvent
NO MATCH in sourceString2

You seem really close:
[^(a-zA-Z)]*(InitializationEvent)[^(a-zA-Z)]*
I think this is what you are looking for. The asterisk provides a match for zero or many of the character or group before it.
EDIT/UPDATE
My apologies on the initial response.
[^a-zA-Z]+(InitializationEvent)[^a-zA-Z]+
My regex is a little rusty, but this will match on any non-alphabet character one or many times prior to the InitializationEvent and after.

How to remove special characters from a string?

I want to remove special characters like:
- + ^ . : ,
from an String using Java.

That depends on what you define as special characters, but try replaceAll(...):
String result = yourString.replaceAll("[-+.^:,]","");
Note that the ^ character must not be the first one in the list, since you'd then either have to escape it or it would mean "any but these characters".
Another note: the - character needs to be the first or last one on the list, otherwise you'd have to escape it or it would define a range ( e.g. :-, would mean "all characters in the range : to ,).
So, in order to keep consistency and not depend on character positioning, you might want to escape all those characters that have a special meaning in regular expressions (the following list is not complete, so be aware of other characters like (, {, $ etc.):
String result = yourString.replaceAll("[\\-\\+\\.\\^:,]","");
If you want to get rid of all punctuation and symbols, try this regex: \p{P}\p{S} (keep in mind that in Java strings you'd have to escape back slashes: "\\p{P}\\p{S}").
A third way could be something like this, if you can exactly define what should be left in your string:
String result = yourString.replaceAll("[^\\w\\s]","");
This means: replace everything that is not a word character (a-z in any case, 0-9 or _) or whitespace.
Edit: please note that there are a couple of other patterns that might prove helpful. However, I can't explain them all, so have a look at the reference section of regular-expressions.info.
Here's less restrictive alternative to the "define allowed characters" approach, as suggested by Ray:
String result = yourString.replaceAll("[^\\p{L}\\p{Z}]","");
The regex matches everything that is not a letter in any language and not a separator (whitespace, linebreak etc.). Note that you can't use [\P{L}\P{Z}] (upper case P means not having that property), since that would mean "everything that is not a letter or not whitespace", which almost matches everything, since letters are not whitespace and vice versa.
Additional information on Unicode
Some unicode characters seem to cause problems due to different possible ways to encode them (as a single code point or a combination of code points). Please refer to regular-expressions.info for more information.

This will replace all the characters except alphanumeric
replaceAll("[^A-Za-z0-9]","");

As described here
http://developer.android.com/reference/java/util/regex/Pattern.html
Patterns are compiled regular expressions. In many cases, convenience methods such as String.matches, String.replaceAll and String.split will be preferable, but if you need to do a lot of work with the same regular expression, it may be more efficient to compile it once and reuse it. The Pattern class and its companion, Matcher, also offer more functionality than the small amount exposed by String.
public class RegularExpressionTest {
public static void main(String[] args) {
System.out.println("String is = "+getOnlyStrings("!&(*^*(^(+one(&(^()(*)(*&^%$##!#$%^&*()("));
System.out.println("Number is = "+getOnlyDigits("&(*^*(^(+91-&*9hi-639-0097(&(^("));
}
public static String getOnlyDigits(String s) {
Pattern pattern = Pattern.compile("[^0-9]");
Matcher matcher = pattern.matcher(s);
String number = matcher.replaceAll("");
return number;
}
public static String getOnlyStrings(String s) {
Pattern pattern = Pattern.compile("[^a-z A-Z]");
Matcher matcher = pattern.matcher(s);
String number = matcher.replaceAll("");
return number;
}
}
Result
String is = one
Number is = 9196390097

Try replaceAll() method of the String class.
BTW here is the method, return type and parameters.
public String replaceAll(String regex,
String replacement)
Example:
String str = "Hello +-^ my + - friends ^ ^^-- ^^^ +!";
str = str.replaceAll("[-+^]*", "");
It should remove all the {'^', '+', '-'} chars that you wanted to remove!

To Remove Special character
String t2 = "!##$%^&*()-';,./?><+abdd";
t2 = t2.replaceAll("\\W+","");
Output will be : abdd.
This works perfectly.

Use the String.replaceAll() method in Java.
replaceAll should be good enough for your problem.

You can remove single char as follows:
String str="+919595354336";
String result = str.replaceAll("\\\\+","");
System.out.println(result);
OUTPUT:
919595354336

If you just want to do a literal replace in java, use Pattern.quote(string) to escape any string to a literal.
myString.replaceAll(Pattern.quote(matchingStr), replacementStr)