RegEx and MySQL basics - replacing a varying part of a string

Question

I have a field in my table that holds a string denoting some object levels, like so:

"<3<"
"<3<5<"
"<3<5<49<"

etc.

I have a function that is to remove a level from such a string, without knowing the position of the level in the string itself. Concretely, I would like to remove "3". The result should be:

"0"
"<5<"
"<5<49<"

If I would, however, want to remove 5, and not 3, the result should be this:

"<3<"
"<3<"
"<3<49<"

Lastly, if I chose to remove 49 instead of 3 or 5, I would like to get this:

"<3<"
"<3<5<"
"<3<5<"

As you can see, the position of the substring that is to be removed varies - sometimes it's the leftmost one, sometimes in the middle, sometimes the rightmost one. What is important after all this is:

If the number I am removing is the only value, enclosed in "less than" signs (as in "<3<" while removing 3), the new result must be 0.
If the number I am removing is not the only value, the only thing that matters is that the final notation stays the same - as in, the entire string must remain enclosed in "less than" symbols, and substrings of multiple "less than" symbols in a row must not happen (as in, "3<<5<" is not allowed).

Is there an easy regex way to handle this with php and mysql, or should I just make 3 manual checks?

P.S. While I may have posed it as such, this is not homework but an actual work issue.

I don't know what objects you're storing in your database but the structure is a pure horror. — Your Common Sense
– Your Common Sense, Commented Oct 24, 2011 at 10:16
This is a categories tree. I have no say in changing the structure itself, unfortunately. The x<y notation indicates a parent-child relationship between category IDs. I myself would have gone with an additional table defining these relationships, but planning would hear none of it because they wanted the entire branch of a subject immediately recognizable without using count(explode("<", $string)) joins. — Swader
– Swader, Commented Oct 24, 2011 at 10:36

Kent · Accepted Answer · 2011-10-24 09:53:22Z

1

for each line two replacements: (for example, you want to remove "3")

replace "^<3<$" -> "0"; 
replace "<3" -> "";

answered Oct 24, 2011 at 9:53

Kent

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1 Comment

Swader Over a year ago

Thanks. While the other answers are technically correct, I wanted a regex answer and I got it.

Luo · Accepted Answer · 2011-10-24 09:40:26Z

1

You can do it in 2 steps. Suppose your input is this

"<3<"
"<3<5<"
"<3<5<49<"

and you want to remove number 3:

Step 1. Since the values always start with "<", you can try to replace "<3" with "". Then the input becomes

"<"
"<5<"
"<5<49<"

Step 2. Replace strings which EQUALS "<" with 0. Then you can get

"0"
"<5<"
"<5<49<"

It's the same if you want to remove 5 or 49. I think you can easily use regex to do these steps.

answered Oct 24, 2011 at 9:40

Luo

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Comments

curial · Accepted Answer · 2011-10-24 10:08:57Z

1

In the first step:

replace "<3(?=<)"

I'ts important to use lookaheads, otherwise you could be replacing something like *<3*4 and that's not what you want.

Second step:

replace "^<$" with "0"

answered Oct 24, 2011 at 10:08

curial

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