Regex - parsing string

Question

Say i have a file which contains lines as follows:

Hayden
y

Suppose i want to manipulate the line which only contains "y" and not the one with Hayden, how would i do this?

So i read in the file and parse it line by line. I want to say if the line contains letters before or after "y" then it's not the line i'm looking for.

I thought i could do the following:

String value = "y"; 
if(strLine.matches("[a-zA-Z]" + value + "[a-zA-Z]"))
{
      don't manipulate line here
}
else
{
   manipulate string here
}

However, this gets "Hayden" as well as "y"

Any ideas?

EDIT

sorry, i should have been more clear, what if i don't mind if there are spaces or symbols in front? or behind? it's specifically the letters that i need to watch out for. For instance, i can't have yh but i can have y=... sorry again

Mark Byers · Accepted Answer · 2012-08-05 22:57:40Z

4

You can use negative lookarounds:

if (strLine.matches("^.*(?<![a-zA-Z])y(?![a-zA-Z]).*$")) { 
    // manipulate string here
}

The anchors are optional but included anyway for clarity.

edited Aug 5, 2012 at 22:57

answered Aug 5, 2012 at 22:44

Mark Byers

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13 Comments

DMor Over a year ago

this is probably the easiest way to show if the line only contains y (and you can also use "equalsIgnoreCase()"

BigBug Over a year ago

sorry, i should have been more clear, what if i don't mind if there are symbols? or spaces in front?

BigBug Over a year ago

this almost works... but for some reason it's not catching y= y+y... hwo would i get around this? as i said, i don't mind there being symbols or spaces... just can't have letters right beside it.. thanks for the response btw

BigBug Over a year ago

@MarkByers sorry, again.. i should have been more clear, i mean right next to "y" there shouldn't be any letters, but any symbol is ok

Mark Byers Over a year ago

@BlueMonster: When you say any letter, do you really mean any letter? Because in your question you write A-Z but there are letters such as Ø that aren't included in A-Z.

|

Bergi · Accepted Answer · 2012-08-05 22:53:16Z

3

You can use:

strLine.matches("^y$")

To ignore symbols, i.e. non-alphanumeric characters, use:

strLine.matches("^\\W*y\\W*$")

edited Aug 5, 2012 at 22:53

Bergi

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answered Aug 5, 2012 at 22:44

Reimeus

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1 Comment

BigBug Over a year ago

sorry, i should have been more clear, what if i don't mind if there are spaces or symbols in front? or behind? it's specifically the letters that i need to watch out for. For instance, i can't have yh but i can have y=... sorry again

Pshemo · Accepted Answer · 2012-08-05 23:05:01Z

1

Maybe this is what you are looking for

String[] lines={"Hayden","y"," y*","y=","y+"," +y..."};
for (String s:lines)
    System.out.println(s+"->"+s.matches("\\W*y\\W*"));

output:

Hayden->false
y->true
 y*->true
y=->true
y+->true
 +y...->true

edited Aug 5, 2012 at 23:05

answered Aug 5, 2012 at 22:51

Pshemo

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2 Comments

BigBug Over a year ago

hmm but what about having something like y= or y+ or +y... i think this would fail in that case... but these lines are ok for me. it's just if there is a letter directly beside y - on either side, that i want to avoid..

BigBug Over a year ago

thanks :) this actually worked. pretty cool stuff man. Thanks again

Blender · Accepted Answer · 2012-08-05 22:45:02Z

0

If you are going to use regex, you need to be a bit more specific:

y matches a y anywhere in the line.
^y$ matches a y that is right before the end of the string (the dollar sign) and right after the beginning of the string (the caret). This lets you match the line that is equal to y.

answered Aug 5, 2012 at 22:45

Blender

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Comments

Bergi · Accepted Answer · 2012-08-05 22:52:26Z

0

Extending the equals approach: Just remove all the character you don't mind, and then check for equality with "y":

if (strLine.replaceAll("[^a-zA-Z]", "").equals("y")) {
    ...
}

answered Aug 5, 2012 at 22:52

Bergi

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3 Comments

BigBug Over a year ago

i can't remove the letters i don't want... they must stay there... i just have to work around them... because i don't want to manipulate those lines...

Bergi Over a year ago

replace() does not manipulate anything, it returns a new string. The condition is to read "if I were to take out everything that is not a-Z and the rest then is just a "y", do..."

BigBug Over a year ago

but i don't want to do anything with the line that contains other letters.... only if the line contains the letter i'm looking for and symbols or spaces... in such a case, i would like to manipulate the string. However, if i remove everything and then check if there is a "y" how do i determine that this is or is not a line containing just the letter i'm looking for and symbols (if any) - although there does not necessarily have to be any symbols...

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