Javascript: Check string to match pattern

Question

I need to test string weather it matches a specific pattern. String is the full file path, for example: C:\some\path\to\folder\folder.jpg or C:\another\file\path\file.jpg

Pattern is: file name without extension should match exact parent folder name, where it is located. File name could be any possible value.

So, in my example only first string matches pattern:

C:\some\path\to\folder\folder.jpg

Can this test be made in javascript using one regular expression?

• user2705585: In your example folder.jpg does have an extention of .jpg. How is condition file name without extension should match exact parent folder name, where it is located satisfied here ?
• Jorge Mejia: try using indexOf method var str = "Hello world, welcome to the universe."; var n = str.indexOf("welcome"); w3schools.com/jsref/jsref_indexof.asp

Answer 1

Yes, you can, with a back reference:

\\([^\\]+)\\1円\.[^.\\]*$

The first group ([^\\]+) captures the folder name, then the back reference (1円) refers back to it saying "the same thing here."

So in the above, we have:

• \\ - matches a literal backslash
• ([^\\]+) - the capture group for the folder name
• \\ - another literal backslash
• 1円 - the back reference to the capture group saying "same thing here"
• \. - a literal . (to introduce the extension)
• [^.\\]* - zero or more extension chars (you may want to change * to + to mean "one or more")
• $ - end of string

On regex101

If you consider C:\some\path\to\folder\folder.test.jpg a valid match (e.g., you think of the extension on folder.test.jpg as being .test.jpg, not .jpg), just remove the . from the [^.\\] near the end.

If you want to allow for files without an extension, perhaps \\([^\\]+)\\1円(?:\.[^.\\]+)?$. The (?:\.[^.\\]+)? is the optional extension.

• Sebastian Proske: I thinky you should add the dot to the last negated character class, otherwise it would match e.g. \folder\folder.test.jpg
• T.J. Crowder: @SebastianProske: You're right, I've fixed it. Although I suppose it depends on your definition of "extension" (is the extension .jpg or .test.jpg). I agree with yours, though.