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Editorial Team
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Asked: 2026-05-27T16:53:43+00:00

I have an Android application in which I’m trying to parse a small sized

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I have an Android application in which I’m trying to parse a small sized XML response coming from server. I have simply just used a regex for doing that, but now I’m facing a problem dealing with optional XML-tags that may or may not be present in the response’s payload. My regex is declared as

final String regex = "^(?=.*<x>(.+)</x>)(?=.*<r>(.+)</r>)?(?=.*<e>(.+)</e>)" +
            "(?=.*<h>(.+)</h>)(?=.*<y>(.+)</y>)(?=.*<n>(.+)</n>).*$";

Note the question mark after the second look ahead group to have the < r > tag be optional. It is causing me an exception when compiling with Pattern.compile:

Pattern p = Pattern.compile(regex);
...
12-19 10:19:21.257: E/AndroidRuntime(2342): Caused by: java.util.regex.PatternSyntaxException: Syntax error U_REGEX_RULE_SYNTAX near index 36:
12-19 10:19:21.257: E/AndroidRuntime(2342): ^(?=.*<x>(.+)</x>)(?=.*<r>(.+)</r>)?(?=.*<e>(.+)</e>)(?=.*<h>(.+)</h>)(?=.*<y>(.+)</y>)(?=.*<n>(.+)</n>).*$
12-19 10:19:21.257: E/AndroidRuntime(2342):                                     ^
12-19 10:19:21.257: E/AndroidRuntime(2342):     at java.util.regex.Pattern.compileImpl(Native Method)
12-19 10:19:21.257: E/AndroidRuntime(2342):     at java.util.regex.Pattern.compile(Pattern.java:400)
12-19 10:19:21.257: E/AndroidRuntime(2342):     at java.util.regex.Pattern.<init>(Pattern.java:383)
12-19 10:19:21.257: E/AndroidRuntime(2342):     at java.util.regex.Pattern.compile(Pattern.java:374)

Without the ?-quantifier it compiles just fine.

I tried also using the same pattern on Windows with desktop java (jdk 1.6.0_24) and it worked as it should (and even parsed my test payload correctly). Are there some known limitations with the native regex implementation on Android or am I just missing something here? I already read the Android documentation regarding Pattern but to no avail. Any help would be appreciated, I wouldn’t like to resort to having to use a dedicated XML parser for something as simple as this.

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1 Answer

  1. Editorial Team

    You usually can’t make a lookahead optional by appending a ?. This is a syntax error in some regex dialects (because a lookahead assertion doesn’t match anything that could be quantified by the ?).

    But you can wrap the lookahead in an optional non-capturing group:

    final String regex = "^(?=.*<x>(.+)</x>)(?:(?=.*<r>(.+)</r>))?(?=.*<e>(.+)</e>)" +
            "(?=.*<h>(.+)</h>)(?=.*<y>(.+)</y>)(?=.*<n>(.+)</n>).*$";
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