Regex to match and limit character classes

Question

I'm not sure if this is possible using Regex but I'd like to be able to limit the number of underscores allowed based on a different character. This is to limit crazy wildcard queries to a search engine written in Java.

The starting characters would be alphanumeric. But I basically want a match if there are more underscores than preceding characters. So

BA_ would be fine but BA___ would match the regex and would get kicked out of the query parser.

Is that possible using Regex?

Answer 1

Yes you can do it. This pattern will succeed only if there are less underscores than letters (you can adapt it with the characters you want) :

^(?:[A-Z](?=[A-Z]*(\\1?+_)))*+[A-Z]+\\1?$

(as Pshemo notices it, anchors are not needed if you use the matches() method, I wrote them to illustrate the fact that this pattern must be bounded whatever the means. With lookarounds for example.)

negated version:

^(?:[A-Z](?=[A-Z]*(\\1?+_)))*\\1?_*$

The idea is to repeat a capture group that contains a backreference to itself + an underscore. At each repetition, the capture group is growing. ^(?:[AZ](?=[AZ]*+(\\\\1?+_)))*+ will match all letters that have a correspondant underscore. You only need to add [AZ]+ to be sure that there is more letters, and to finish your pattern with \\\\1? that contains all the underscores (I make it optional, in case there is no underscore at all).

Note that if you replace [AZ]+ with [AZ]{n} in the first pattern, you can set exactly the number of characters difference between letters and underscores.

---

To give a better idea, I will try to describe step by step how it works with the string ABC-- (since it's impossible to put underscores in bold, i use hyphens instead) :

 In the non-capturing group, the first letter is found 
BC--        ^(?:(?=[A-Z]*(\1?+-)))*+[A-Z]+\1?$
 let's enter the lookahead (keep in mind that all in the lookahead is only
 a check and not a part of the match result.)
BC--        ^(?:[A-Z][A-Z]*(\1?+-))*+[A-Z]+\1?$
--        ^(?:[A-Z](?=(\1?+-)))*+[A-Z]+\1?$

 the first capturing group is encounter for the first time and its content is not
 defined. This is the reason why an optional quantifier is used, to avoid to make
 the lookahead fail. Consequence: \1?+ doesn't match something new.
--        ^(?:[A-Z](?=[A-Z]*(-)))*+[A-Z]+\1?$

 the first hyphen is matched. Once the capture group closed, the first capture
    group is now defined and contains one hyphen. 
-        ^(?:[A-Z](?=[A-Z]*(\1?+)))*+[A-Z]+\1?$

The lookahead succeeds, let's repeat the non-capturing group.
BC--        ^[A-Z](?=[A-Z]*(\1?+-))[A-Z]+\1?$

The second letter is found
C--        ^(?:(?=[A-Z]*(\1?+-)))*+[A-Z]+\1?$
 We enter the lookahead
--        ^(?:[A-Z](?=(\1?+-)))*+[A-Z]+\1?$

but now, things are different. The capture group was defined before and
 contains an hyphen, this is why \1?+ will match the first hyphen.
-        ^(?:[A-Z](?=[A-Z]*(-)))*+[A-Z]+\1?$
 the literal hyphen matches the second hyphen in the string. And now the
 capture group 1 contains the two hypens. The lookahead succeeds.
        ^(?:[A-Z](?=[A-Z]*(\1?+)))*+[A-Z]+\1?$

We repeat one more time the non capturing group.
--        ^(?:(?=[A-Z]*(\1?+-)))*+[A-Z]+\1?$
 In the lookahead. There is no more letters, it's not a problem, since
 the * quantifier is used.
--        ^(?:[A-Z](?=(\1?+-)))*+[A-Z]+\1?$
 \\1?+ matches now two hyphens.
        ^(?:[A-Z](?=[A-Z]*(-)))*+[A-Z]+\1?$

but there is no more hyphen in the string for the literal hypen and 
 The lookahead fails. Thus the third repetition of the non-capturing group too!
C--        ^(?:[A-Z](?=[A-Z]*(\1?+-)))*+[A-Z]+\1?$
 ensure that there is at least one more letter.
--        ^(?:[A-Z](?=[A-Z]*(\1?+-)))*+\1?$

We match the end of the string with the backreference to capture group 1 that
 contains the two hyphens. Note that the fact that this backreference is optional
 allows the string to not have hyphens at all. 
        ^(?:[A-Z](?=[A-Z]*(\1?+-)))*+[A-Z]+$
 This is the end of the string. The pattern succeeds.
        ^(?:[A-Z](?=[A-Z]*(\1?+-)))*+[A-Z]+\1?

---

Note: The use of the possessive quantifier for the non-capturing group is needed to avoid false results. (Where you can observe a strange behavior, that can be useful.)

Example: ABC--- and the pattern: ^(?:[AZ](?=[AZ]*(\\1?+-)))*[AZ]+\\1?$ (without the possessive quantifier)

Then the letter C is found:
---     ^(?:[A-Z](?=[A-Z]*(\1?+-)))*\1?$
 And the three hyphens
     ^(?:[A-Z](?=[A-Z]*(\1?+-)))*[A-Z]+$
 The pattern succeeds
     ^(?:[A-Z](?=[A-Z]*(\1?+-)))*[A-Z]+\1?

Question: How many hyphens are in the capture group now?
Answer: Always three!

If the repeated non-capturing group gives a letter back, the capture group contains always three hyphens (as the last time the capture group has been read by the regex engine).This is counter-intuitive, but logical.

$pattern = <<<'EOD'
~
 (?(DEFINE)
     (?<neutral> (?: _ \g<neutral>?+ [A-Z] | [A-Z] \g<neutral>?+ _ )+ )
 )

 \A (?: \g<neutral> | _ )+ \z
~x
EOD;

var_dump(preg_match($pattern, '____ABC_DEF___'));

---

Robby Pond asked me in comments how to find strings that have more underscores than letters (all that is not an underscore) . The best way is obviously to count the numbers of underscores and to compare with the string length. But about a full regex solution, it is not possible to build a pattern for that with Java since the pattern needs to use the recursion feature. For example you can do it with PHP:

 $pattern = <<<'EOD' ~ (?(DEFINE) (?<neutral> (?: _ \\g<neutral>?+ [AZ] | [AZ] \\g<neutral>?+ _ )+ ) ) \\A (?: \\g<neutral> | _ )+ \\z ~x EOD; var_dump(preg_match($pattern, '____ABC_DEF___')); 

Answer 2

Its not possible in singular regular expression.

i) Logic needs to be implemented to get number of characters before underscores(regular expression should be written to get characters word before underscore).

ii) And validate result (number of characters - 1) = number of semicolons followed(regular expression which returns stream of underscores followed by characters).

Answer 3

Edit: Dang! I just noticed that you need this for java. Anyways...I leave it here if someone from the .Net world stumbles upon this post.

You can use Balancing Groups if you are using .Net:

^(?:(?<letter>[^_])|(?<-letter>_))*(?(letter)(?=)|(?!))$

The .net regex engine has the ability to maintain all captured patterns in the captured groups. In other flavors the captured group would always contain the last matched pattern but in .net all previous matches are contained in a capture collection for your use. Also the .net engine has the ability to push and pop to the stack of the captured groups using the ?<group-name> , ?<-group-name> constructs. These two handy constructs can be utilized to match pairs of paranthesis, etc.

In the above regex, the engine starts from the start of the string and tries to match anything other than "_". This of course can be changed to whatever works for you(eg [AZ][az] ). The alternation basically means either match [^\\_] or [\\_] and doing so either push or pop from the captured group.

The latter part of the regex is a conditional (?(group-name)true|false) . It basically says, if the group still exists(more pushes than pops), then do the true section and if not do the false section. The easiest way to make the pattern match is to use an empty positive look ahead: (?=) and the easiest way to make it fail is (?!) which is a negative lookahead.