Create regex of string

Question

I have input string which I want to convert to regex and then compare against another string.

String inputString = "https://*.mlc.net/Gateway/api/Transmissions/State (body 0 bytes)";

String strToMatch = "https://abc.mlc.net/Gateway/api/Transmissions/State (body 0 bytes)";

I have done it like:

String regex = inputString.replace("*", "(.*)");

strToMatch.matches(regex)

but it fails. Can someone please explain the code?

I know backslashes are required before paranthesis but would require code which will work for all characters in the input string.

You also need to escape the characters that have a meaning in a regexp. — Maurice Perry
– Maurice Perry, Commented Feb 18, 2019 at 9:27
In your input string '(' and ')' should be escaped. inputString = "https://*.mlc.net/Gateway/api/Transmissions/State \\(body 0 bytes\\)"; should work for the given example. — Eritrean
– Eritrean, Commented Feb 18, 2019 at 9:36

Sweeper · Accepted Answer · 2019-02-18 09:37:07Z

2

Your regex doesn't match because there are some regex metacharacters in your input string. Namely, the regex engine interprets the parentheses around body 0 bytes as a capture group instead of literal parentheses. You want everything in the input string to be matched literally, except the (.*) parts, right?

You can split the input string with *, then for each part, call Pattern.quote to escape all the regex meta characters. Finally, you join the parts together with (.*):

private static String createRegex(String inputString) {
    String[] parts = inputString.split("\\*");
    return Arrays.stream(parts).map(Pattern::quote).collect(Collectors.joining("(.*)"));
}

answered Feb 18, 2019 at 9:37

Sweeper

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4 Comments

user3770039 Over a year ago

Right, but can you please confirm if this will work for all characters in the input string which have meaning in regex?

Sweeper Over a year ago

@user3770039 Yes it will, according to the docs.

user3770039 Over a year ago

Thanks, will this work if input string does not contain *?

Sweeper Over a year ago

@user3770039 Yes, it will create a regex that matches exactly the input string.

Maurice Perry · Accepted Answer · 2019-02-18 09:36:28Z

You also need to escape the characters that have a meaning in a regexp:

    String regex = formatRe(inputString);

...

private static String formatRe(String inputString) {
    StringBuilder buf = new StringBuilder();
    for (char c: inputString.toCharArray()) {
        switch (c) {
            case '*':
                buf.append("(.*)");
                break;
            case '.':
            case '(':
            case ')':
            case '[':
            case ']':
                // ...
                buf.append('\\');
                buf.append(c);
                break;
            case '\t':
                buf.append("\\t");
                break;
            default:
                buf.append(c);
                break;
        }
    }
    return buf.toString();
}

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