java 如何检查字符串是否为正则表达式
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How to check if the string is a regular expression or not
提问by Saurabh Kumar
I have a string. How I can check if the string is a regular expression or contains regular expression or it is a normal string?
我有一个字符串。如何检查字符串是正则表达式或包含正则表达式还是普通字符串？
回答by Joachim Sauer
The only reliable check you could do is if the Stringis a syntactically correct regular expression:
您可以做的唯一可靠检查是 是否String是语法正确的正则表达式：
boolean isRegex;
try {
Pattern.compile(input);
isRegex = true;
} catch (PatternSyntaxException e) {
isRegex = false;
}
Note, however, that this will result in trueeven for strings like Hello Worldand I'm not a regex, because technically they are valid regular expressions.
但是请注意，true即使对于Hello World和这样的字符串，这也会导致I'm not a regex，因为从技术上讲，它们是有效的正则表达式。
The only cases where this will return falseare strings that are not valid regular expressions, such as [unclosed character classor (unclosed groupor +.
这将返回的唯一情况false是不是有效正则表达式的字符串，例如[unclosed character classor(unclosed group或+。
回答by hoipolloi
This is ugly but will detect simple regular expressions (with the caveat they must be designed for Java i.e. have the relevant back-slash character escaping).
这是丑陋的，但会检测简单的正则表达式（注意它们必须是为 Java 设计的，即具有相关的反斜杠字符转义）。
public boolean isRegex(final String str) {
try {
java.util.regex.Pattern.compile(str);
return true;
} catch (java.util.regex.PatternSyntaxException e) {
return false;
}
}
回答by JoséMi
Maybe you'd try to compile that regular expression using regexp package from Apache ( http://jakarta.apache.org/regexp/) and, if you get an exception then that's not a valid regexp so you'd say it's a normal string.
也许您会尝试使用来自 Apache ( http://jakarta.apache.org/regexp/) 的regexp 包编译该正则表达式，如果出现异常，则这不是有效的正则表达式，因此您会说这是正常的细绳。
boolean validRE = true;
try {
RE re = new RE(stringToCheck);
} catch (RESyntaxException e) {
validRE = false;
}
Obviously, the user would have typed an invalid regexp and you'd be handling it as a normal string.
显然，用户输入了无效的正则表达式，而您将其作为普通字符串进行处理。
回答by Sam Holder
there is no difference between a 'normal' sting and a regular expression. A regular expression is just a normal string which is used as a pattern to match occurrences of the pattern in another string.
“正常”刺痛和正则表达式之间没有区别。正则表达式只是一个普通字符串，用作模式来匹配另一个字符串中出现的模式。
As others have pointed out, it is possible that the string might not be a validregular expression, but I think that is the only check you can do. If it is valid then there is no way to know if it is a regular expression or just a normal string because it will be a regular expression
正如其他人指出的那样，该字符串可能不是有效的正则表达式，但我认为这是您可以做的唯一检查。如果它有效，则无法知道它是正则表达式还是普通字符串，因为它将是正则表达式
It is just a normal string which is interpreted in a specific way by the regex engine.
它只是一个普通字符串，由正则表达式引擎以特定方式解释。
for example "blah" is a regular expression which will only match the string "blah" where ever it occurs in another string.
例如“blah”是一个正则表达式，它只会匹配字符串“blah”出现在另一个字符串中的地方。
When looked at this way, you can see that a regular expression does not need to contain any of the 'special characters' that do more advanced pattern matching, and it will only match the string in the pattern
以这种方式查看时，您可以看到正则表达式不需要包含任何进行更高级模式匹配的“特殊字符”，它只会匹配模式中的字符串
回答by Anonymous
/**
* If input string is a regex, matches will always return a false.
*/
public boolean isRegex(final String str) {
return str != null ? !str.matches(str) : false;
}
