Let’s now continue dissecting the regular expression: ” .*\.doc“
In Part 1, I introduced you to literal characters and explained that the characters ‘.‘, ‘d‘, ‘o‘, and ‘c‘ of the .doc portion of that regex are actually literal characters. I also mentioned that the metacharacter ‘\’ lets our server know that any metacharacter placed right after it should be treated as a literal character.
That leaves us with the two metacharacters: ‘.‘ and ‘*‘. Let’s have a closer look at these two now.
The metacharacters dot (.) and asterisk (*) and how to match specific file types
When the metacharacter ‘.‘ (assuming it is not preceded by a ‘\‘) is compared with any single character, the two will automatically match. It doesn’t matter whether the other character is a number, a letter or any other character. The server will simply treat the pair as a match. There are a few exceptions, but we won’t be using those here, or at least at this stage.
The ‘*‘, on the other hand, is not compared with another character. Instead, it is used to tell the server to try to match the character that precedes it from zero to as many times as possible.
If we want to tell the server to match a character at least one time, we can use “+” instead of “*”. This enforces that the character preceding it must appear at least once.
In our examples, the character that preceded the ‘*‘ was the ‘.‘, which, as we mentioned, automatically matches any character. So let’s see what the result would be if we tried to match afile.doc with the regex “.*\.doc“.
Obviously, the last four characters .doc already match, so we can take them out. That leaves us with afile and ‘.*‘. Upon seeing the ‘.*‘, the server knows that it should try to match ‘.‘ as many times as possible. It starts by comparing it with ‘a‘. Since ‘.‘ can match any character, it can certainly match with ‘a‘. Because of the ‘*‘, the server proceeds to compare ‘.‘ with ‘f‘. Again, they naturally match.
This goes on until ‘.‘ is compared with ‘e‘. Of course, they too match. Since all of their characters match, the server concludes that afile.doc and the regex “.*\.doc” match.
If afile.doc is the value stored in the variable LocalPath, then the trigger condition:
LocalPath ~ “.*\.doc” would return TRUE.
You can apply the same concept to the regexes: “.*\.txt“, “.*\.pdf “, “.*\.jpg “ and so on.
Now you know what regex to use when you want a trigger condition to apply to a specific file type.
More examples using “.*”
The “.*” doesn’t always have to be placed at the beginning of a regex. Consider the regex “image.*a\.jpg“. Since image and a.jpg (again, remember the ‘.‘ here is considered a literal character) are made up of literal characters, this regex will match any jpg file with a filename beginning with “image” and ending in ‘a.jpg‘. So,
- image001a.jpg,
- image12xpza.jpg and
- imageABCa.jpg
all match with the regex “image.*a\.jpg “.
Note that even if there are no characters between “image” and ‘a‘, such as imagea.jpg, the match will still hold. That’s because, if you recall, ‘*‘ will instruct the server to try to match ‘.‘ from zero to as many times as possible.
Feeling empowered already? You ain’t seen nothing yet.
Why don’t you test the filename “imageABCA.jpg” against our regex? It’s almost like “imageABCa.jpg“, which matched earlier. Well, almost but not quite. Regexes used in Trigger conditions and in “If Actions” are case-sensitive. We can modify our regex a bit to make it case-insensitive for specific characters, like this:
“image.*[aA]\.jpg“
To make the whole regex case insensitive (and not just the “a” at the end of the file name), we can set the “(?i)” flag, like this:
“(?i)Imagea\.jpg”
That should match that last filename (imageABCA.jpg) as well as those three other filenames we tried earlier. It also introduces us to a new pair of special characters.
Notice that the same is not true for some of the regex actions (such as “Copy/Move Regex File” and “Regex File Upload/Download” variants). The regex used in those transfer actions will be case-insensitive. We will go over those later in this post.
Introducing character classes or character sets and case-insensitive regexes
A pair of square brackets, including the characters they enclose, is collectively called a character class or character set. When the server finds a character class in its most simple form, i.e., a pair of square brackets and two or more alphanumeric characters, the server will try to find a single-character match using only one of the characters found inside.
For example, the regex image[aAbBcC]\.jpg can match any of the following, which in a way deals with the problem of case sensitivity:
- imagea.jpg
- imageA.jpg
- imageb.jpg
- imageB.jpg
- imagec.jpg
- imageC.jpg
but it cannot match:
- imaged.jpg
- imageaa.jpg
- imagez.jpg
- imageab.jpg
You might be wondering why imageab.jpg or imageaa.jpg do not match. Remember that when the server tries to find a single character match from among the characters in a character class, it will only pick one, and only one, character. Once the ‘a’ in the character class has already matched, ‘b’ can no longer match. And since the regex does not have a literal character ‘b’ or another token to match it right after the character class, the ‘b’ in imageab.jpg fails to match at all.
There are two ways to accommodate imageab.jpg if we want to employ only what we’ve discussed so far.
The first one is to simply insert the literal character ‘b‘ between the character class and the ‘\.jpg‘ like this:
image[aAbBcC]b\.jpg
The other option would be to place an ‘*‘ right after the character class, matching a single character from the class between zero and as many times as possible:
image[aAbBcC]*\.jpg
This will tell the server to match any of the characters in the character class from zero to as many times as possible. But be careful, because the second option will also match the following:
- imageabcABC.jpg
- image.jpg
This is important to note, as this may not be what you would like to achieve. Using “+” enforces that one of the characters in the class must be present at least once.
image[aAbBcC]+\.jpg
This is enough for the regex to not match image.jpg, but still match imageA.jpg, imageAB.jpg and so on.
As mentioned earlier, if the whole regex can be case-insensitive, we just need to add the “(?i)” flag at the start of it.
(?i)image[abc]+\.jpg
The flag will now cause the regex above to match:
- ImAgEbcABC.jpg
- IMAGEa.jpg
- imagea.JPG
That’s right! The file name extension also has to be taken into consideration for case sensitivity. It doesn’t happen often, but remember to take that into consideration.
One last topic: File transfer Actions used in Triggers that make use of regex are case-insensitive by default. Some examples are:
- Trading Partner Regex File Upload
- Trading Partner Regex File Download
- Copy Regex File
- Move Regex File
If any of those actions use the example regex image[abc]+\.jpg, they will match:
- imageA.jpg
- IMAGEccc.jpg
This might not be what you want. If you want the file actions to be exclusively case sensitive, we have to add a different flag at the start of the regex to enforce that, this time being the “(?-i)” flag.
If we use the regex (?-i)image[abc]+\.jpg, the file actions will only act on files that match the exact case of the regex, such as:
- imagea.jpg
- imageabc.jpg
And the action will now ignore any files named like:
- imageA.jpg
- IMAGEccc.jpg
Numbers and number ranges in character classes
Character classes don’t just match letters of the alphabet. They also match numbers or a range of numbers.
To match all .doc files that end in the numbers 1, 3, or 5, you use “.*[135]\.doc“. So, the following files will match:
- word 1.doc
- myletter 3.doc
- myletter13 5.doc
- report3 3.doc
I highlighted the number on each filename that matched a number in the character class to emphasize that only one character is matched in a character class. All preceding characters on each of those filenames were, of course, matched by ‘.*‘.
To match a range of numbers, just use a hyphen. So let’s say you want to match all .doc files that begin with any number from 6 to 9. You can use this regex: “[6-9].*\.doc“. This should match the following files:
- 6Abatch2.doc
- 8245file.doc
- 9.doc
You can even combine letters, numbers and ranges of numbers like this: “[6-9jpv124]“. I leave it to you to try this out.
Regexes for multiple file types
So far, we’ve only been talking about regexes whose practical applications only apply to one type of file at a time. What if we wanted to match all jpg, doc and xls files?
For this, you’ll need the vertical bar ‘|‘. The vertical bar will allow the server to match one regex from a selection of regexes. So, if the individual regular expressions for matching jpg, doc and xls files are:
- “.*\.jpg“
- “.*\.doc“
- “.*\.xls“
then the combined regex would simply be: “.*\.jpg| .*\.doc| .*\.xls“.
There are lots of areas in JSCAPE MFT Server by Redwood where regular expressions can be employed. One of them is the DLP (Data Loss Prevention) module. The regular expressions there are quite more complicated than those that we’ve seen here so far, so I’ll cover that in a separate post. Hope to see you there.
