Write regular expressions in pure Java
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Java Regex Builder

Write regexes as plain Java code. Unlike opaque regex strings, commenting your expressions and reusing regex fragments is straightforward.

The regex-builder library is implemented as a light-weight wrapper around java.util.regex. It consists of three main components: the expression builder Re, its fluent API equivalent FluentRe, and the character class builder CharClass. The components are introduced in the examples below as well as in the API overview tables at the end of this document.

There's a discussion of this project over on the Java reddit.

Maven dependency




import com.github.sgreben.regex_builder.CaptureGroup;
import com.github.sgreben.regex_builder.Expression;
import com.github.sgreben.regex_builder.Pattern;
import static com.github.sgreben.regex_builder.CharClass.*;
import static com.github.sgreben.regex_builder.Re.*;

Apache log

  • Regex string: (\\S+) (\\S+) (\\S+) \\[([\\w:/]+\\s[+\\-]\\d{4})\\] \"(\\S+) (\\S+) (\\S+)\" (\\d{3}) (\\d+)
  • Java code:
CaptureGroup ip, client, user, dateTime, method, request, protocol, responseCode, size;
Expression token = repeat1(nonWhitespaceChar());

ip = capture(token);
client = capture(token);
user = capture(token);
dateTime = capture(sequence(
  repeat1(union(wordChar(),':','/')),  whitespaceChar(), oneOf("+\\-"), repeat(digit(), 4)
method = capture(token);
request = capture(token);
protocol = capture(token);
responseCode = capture(repeat(digit(), 3));
size = capture(number());

Pattern p = Pattern.compile(sequence(
  ip, ' ', client, ' ', user, " [", dateTime, "] \"", method, ' ', request, ' ', protocol, "\" ", responseCode, ' ', size

Note that capture groups are plain java objects - no need to mess around with group indices or string group names. You can use the expression like this:

String logLine = " - - [21/Jul/2014:9:55:27 -0800] \"GET /home.html HTTP/1.1\" 200 2048";
Matcher m = p.matcher(logLine);


assertEquals("", m.group(ip));
assertEquals("-", m.group(client));
assertEquals("-", m.group(user));
assertEquals("21/Jul/2014:9:55:27 -0800", m.group(dateTime));
assertEquals("GET", m.group(method));
assertEquals("/home.html", m.group(request));
assertEquals("HTTP/1.1", m.group(protocol));
assertEquals("200", m.group(responseCode));
assertEquals("2048", m.group(size));

Or, if you'd like to rewrite the log to a simpler "ip - request - response code" format, you can simply do

String result = m.replaceFirst(replacement(ip, " - ", request, " - ", responseCode));

Apache log (fluent API)

The above example can also be expressed using the fluent API implemented in FluentRe. To use it, you have import it as

import static com.github.sgreben.regex_builder.CharClass.*;
import com.github.sgreben.regex_builder.FluentRe;
CaptureGroup ip, client, user, dateTime, method, request, protocol, responseCode, size;
FluentRe nonWhitespace = FluentRe.match(nonWhitespaceChar()).repeat1();

ip = nonWhitespace.capture();
client = nonWhitespace.capture();
user = nonWhitespace.capture();
dateTime = FluentRe
    .match(union(wordChar(), oneOf(":/"))).repeat1()
method = nonWhitespace.capture();
request = nonWhitespace.capture();
protocol = nonWhitespace.capture();
responseCode = FluentRe.match(digit()).repeat(3).capture();
size = FluentRe.match(digit()).repeat1().capture();

Pattern p = FluentRe.match(beginInput())
    .then(ip).then(' ')
    .then(client).then(' ')
    .then(user).then(" [")
    .then(dateTime).then("] \"")
    .then(method).then(' ')
    .then(request).then(' ')
    .then(protocol).then("\" ")
    .then(responseCode).then(' ')


  • Regex string: (\d\d\)/(\d\d)\/(\d\d\d\d) (\d\d):(\d\d):(\d\d)
  • Java code:
Expression twoDigits = repeat(digit(), 2);
Expression fourDigits = repeat(digit(), 4);
CaptureGroup day = capture(twoDigits);
CaptureGroup month = capture(twoDigits);
CaptureGroup year = capture(fourDigits);
CaptureGroup hour = capture(twoDigits);
CaptureGroup minute = capture(twoDigits);
CaptureGroup second = capture(twoDigits);
Expression dateExpression = sequence(
  day, '/', month, '/', year, ' ', // DD/MM/YYY
  hour, ':', minute, ':', second,    // HH:MM:SS

Use the expression like this:

Pattern p = Pattern.compile(dateExpression)
Matcher m = p.matcher("01/05/2015 12:30:22");
assertEquals("01", m.group(day));
assertEquals("05", m.group(month));
assertEquals("2015", m.group(year));
assertEquals("12", m.group(hour));
assertEquals("30", m.group(minute));
assertEquals("22", m.group(second));

Hex color

  • Regex string: #([a-fA-F0-9]){3}(([a-fA-F0-9]){3})?
  • Java code:
Expression threeHexDigits = repeat(hexDigit(), 3);
CaptureGroup hexValue = capture(
    threeHexDigits,              // #FFF  
    optional(threeHexDigits)  // #FFFFFF
Expression hexColor = sequence(
  '#', hexValue

Use the expression like this:

Pattern p = Pattern.compile(hexColor);
Matcher m = p.matcher("#0FAFF3 and #1bf");
assertEquals("0FAFF3", m.group(hexValue));
assertEquals("1bf", m.group(hexValue));

Reusing expressions

To reuse an expression cleanly, it should be packaged as a class. To access the capture groups contained in the expression, each capture group should be exposed as a final field or method.

To allow the resulting object to be used as an expression, regex-builder provides a utility class ExpressionWrapper, which exposes a method setExpression(Expression expr) and implements the Expresssion interface.

import com.github.sgreben.regex_builder.ExpressionWrapper;

To use the class, simply extend it and call setExpression in your constructor or initialization block. You can then pass it to any regex-builder method that expects an Expression.

Reusable Apache log expression

Using ExpressionWrapper, we can package the Apache log example above as follows:

public class ApacheLog extends ExpressionWrapper {
    public final CaptureGroup ip, client, user, dateTime, method, request, protocol, responseCode, size;

        Expression nonWhitespace = repeat1(CharClass.nonWhitespaceChar());
        ip = capture(nonWhitespace);
        client = capture(nonWhitespace);
        user = capture(nonWhitespace);
        dateTime = capture(sequence(
            repeat1(union(wordChar(), ':', '/')),
            repeat(digit(), 4)
        method = capture(nonWhitespace);
        request = capture(nonWhitespace);
        protocol = capture(nonWhitespace);
        responseCode = capture(repeat(CharClass.digit(), 3));
        size = capture(repeat1(CharClass.digit()));

        Expression expression = sequence(
            ip, ' ', client, ' ', user, " [", dateTime, "] \"", method, ' ', request, ' ', protocol, "\" ", responseCode, ' ', size,

We can then use instances of the packaged expression like this:

public static boolean sameIP(String twoLogs) {
    ApacheLog log1 = new ApacheLog();
    ApacheLog log2 = new ApacheLog();
    Pattern p = Pattern.compile(sequence(
        log1, ' ', log2
    Matcher m = p.matcher(twoLogs);
    return m.group(log1.ip).equals(m.group(log2.ip));


Expression builder

Builder method java.util.regex syntax
repeat(e, N) e{N}
repeat(e) e*
repeat(e).possessive() e*+
repeatPossessive(e) e*+
repeat1(e) e+
repeat1(e).possessive() e++
repeat1Possessive(e) e++
optional(e) e?
optional(e).possessive() e?+
optionalPossessive(e) e?+
capture(e) (e)
positiveLookahead(e) (?=e)
negativeLookahead(e) (?!e)
positiveLookbehind(e) (?<=e)
negativeLookbehind(e) (?<!e)
backReference(g) \g
separatedBy(sep, e) (?:e((?:sep)(?:e))*)?
separatedBy1(sep, e) e(?:(?:sep)(?:e))*
choice(e1,...,eN) (?:e1|...| eN)
sequence(e1,...,eN) e1...eN
string(s) \Qs\E
word() \w+
number() \d+
whitespace() \s*
whitespace1() \s+
CaptureGroup g = capture(e) (?g e)

CharClass builder

Builder method java.util.regex syntax
range(from, to) [from-to]
range(f1, t1, ..., fN, tN) [f1-t1f2-t2...fN-tN]
oneOf("abcde") [abcde]
union(class1, ..., classN) [[class1]...[classN]]
complement(class1) [^[class1]]
anyChar() .
digit() \d
nonDigit() \D
hexDigit() [a-fA-F0-9]
nonHexDigit() [^[a-fA-F0-9]]
wordChar() \w
nonWordChar() \W
wordBoundary() \b
nonWordBoundary() \B
whitespaceChar() \s
nonWhitespaceChar() \S
verticalWhitespaceChar() \v
nonVerticalWhitespaceChar() \V
horizontalWhitespaceChar() \h
nonHorizontalWhitespaceChar() \H