Why is this regex not working properly?

Question

I have this regex: ^\/\* to check and see if a file contains those two characters at the beginning. I'm iterating over many c++ source files trying to see which of them contain that. The problem is, that if a file contains this:

#include <source.h>

/* this is a comment */

this also matches the regex. I don't understand why, as the regex doesn't have the multiline flag on.

Here's the code for the regex:

multi = /^\/\*/

Why isn't this matching only at the beginning of the text? Here's basically everything I'm doing:

data = File.read(filename)
if data =~ multi
   puts "file starts with multiline header"
end

Andomar · Accepted Answer · 2009-11-16 12:54:21Z

5

In Ruby ^ matches after every newline. Use \A to match only at the start of the entire string:

multi = /\A\/\*/

answered Nov 16, 2009 at 12:54

Andomar

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2 Comments

Geo Over a year ago

This kind of sux. Is this Ruby only behaviour, or do all the languages have this?

Andomar Over a year ago

Most languages have a switch called "multiline mode" that causes them to behave like this, but Ruby is the only one I know that does it by default. Perl's multiline mode looks like /^test/m

Tim Pietzcker · Accepted Answer · 2009-11-16 13:02:50Z

3

Use \A (beginning of string) instead of ^ (beginning of line).

The interpretation of ^ is not completely consistent between flavors. Sometimes you need to set a mode modifier for multi-line strings, but not always. \A is consistent (although not available in all flavors, but most of them. Exceptions are XML, POSIX ERE/BREs and a few others).

edited Nov 16, 2009 at 13:02

answered Nov 16, 2009 at 12:55

Tim Pietzcker

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2 Comments

Geo Over a year ago

Is this the same in Perl/Python?

Tim Pietzcker Over a year ago

In Perl/Python, behaviour of ^ depends on the mode modifiers used (/m in Perl or re.MULTILINE in Python).

Svante · Accepted Answer · 2009-11-16 13:19:23Z

1

Why use a regular expression?

multi = "/*"
data = File.read(filename)
if data[0..2] == multi
   puts "file starts with multiline header"
end

answered Nov 16, 2009 at 13:19

Svante

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1 Comment

Geo Over a year ago

Because additional spaces may be present before the comment start. The regex was just to find out why Ruby behaved like that.

Pherrymason · Accepted Answer · 2009-11-16 13:00:12Z

0

I don't know of ruby internals, but try this:

/^[^a-zA-Z#<>]/*/

The first part ensures that any valid character is not found before your multiline comment. Please, note that [^a-zA-Z#<>] is just an example, you should complete it with a valid combination.

answered Nov 16, 2009 at 13:00

Pherrymason

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