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I am currently trying to learn how to use regular expressions so please bear with my simple question. For example, say I have an input file containing a bunch of links separated by a newline:
www.foo.com/Archives/monkeys.htm
Description of Monkey's website.www.foo.com/Archives/pigs.txt
Description of Pig's website.www.foo.com/Archives/kitty.txt
Description of Kitty's website.www.foo.com/Archives/apple.htm
Description of Apple's website.
If I wanted to get one website along with its description, this regex seems to work on a testing tool: .*www.*\\s.*Pig.*
However, when I try running it within my code it doesn't seem to work. Is this expression correct? I tried replacing "\s" with "\n" and it doesn't seem to work still.
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"\n" matches a newline character.
By default in most regex engines, . doesn't match newline characters, so the matching stops at the end of each logical line. If you want . to match really everything, including newlines, you need to enable "dot-matches-all" mode in your regex engine of choice (for example, add re. DOTALL flag in Python, or /s in PCRE.
String regex = "\\."; Notice that the regular expression String contains two backslashes after each other, and then a . . The reason is, that first the Java compiler interprets the two \\ characters as an escaped Java String character. After the Java compiler is done, only one \ is left, as \\ means the character \ .
\\. matches the literal character . . the first backslash is interpreted as an escape character by the Emacs string reader, which combined with the second backslash, inserts a literal backslash character into the string being read. the regular expression engine receives the string \.
The lines are probably separated by \r\n in your file. Both \r (carriage return) and \n (linefeed) are considered line-separator characters in Java regexes, and the . metacharacter won't match either of them. \s will match those characters, so it consumes the \r, but that leaves .* to match the \n, which fails. Your tester probably used just \n to separate the lines, which was consumed by \s.
If I'm right, changing the \s to \s+ or [\r\n]+ should get it to work. That's probably all you need to do in this case, but sometimes you have to match exactly one line separator, or at least keep track of how many you're matching. In that case you need a regex that matches exactly one of any of the three most common line separator types: \r\n (Windows/DOS), \n (Unix/Linus/OSX) and \r (older Macs). Either of these will do:
\r\n|[\r\n] \r\n|\n|\r Update: As of Java 8 we have another option, \R. It matches any line separator, including not just \r\n, but several others as defined by the Unicode standard. It's equivalent to this:
\r\n|[\n\x0B\x0C\r\u0085\u2028\u2029] Here's how you might use it:
(?im)^.*www.*\R.*Pig.*$ The i option makes it case-insensitive, and the m puts it in multiline mode, allowing ^ and $ to match at line boundaries.
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