How to use start of line/end of line signs (^ or $) in different contexts?

Question

While doing some small regex task I came upon this problem. I have a string that is a list of tags that looks e.g like this:
foo,bar,qux,garp,wobble,thud

What I needed to do was to check if a certain tag, e.g. 'garp' was in this list. (What it finally matches is not really important, just if there is a match or not.)

My first and a bit stupid try at this was to use the following regex:
[^,]garp[,$]

My idea was that before 'garp' there should either be the start of the line/string or a comma, after 'garp' there should be either a comma or the end of the line/string.

Now, it is instantly obvious that this regex is wrong: Both ^ and $ change their behaviour in the context of the character class [ ].

What I finally came up with is the following:
^garp$|^garp,|,garp,|,garp$

This regex just handles the 4 cases one by one. (Tag at beginning of list, in the center, at the end, or as the only element of the list.) The last regex is somehow a bit ugly in my eyes and just for funs sake I'd like to make it a bit more elegant.

Is there a way how the start of line/end of line characters (^ and $) can be used in the context of character classes?

EDIT: Ok, some more info was wished so here it is: I'm using this within an Oracle SQL statement. This sadly does not allow any look-around assertions but as I'm only interested if there is a match or not (and not what is matched) this does not really affect me here. The tags can contain non-alphabetical characters like - or _ so \bgarp\b would not work. Also one tag can contain an other tag as SilentGhost said, so /garp/ doesnt work either.

Contorted

Maybe I'm minunderstanding, but I think you're overlooking the obvious. The regex /garp/ on its own should be fine. It will tell you if the string 'garp' is matched anywhere in the line. A few questions: what language are you using? What is your larger goal? (Do you need to extract matches for example?) Why do you think that you need to match every particular way that 'garp' may appear in the line? – Boiler

@Telemachus: /garp/ would also find agarpor for example. – Bilbo

@SilentGhost: Yup, that's true (and stupid of me). I misunderstood the OP's "What it finally matches is not really important." – Boiler

Answer 1

You can't use ^ and $ in character classes in the way you wish - they will be interpreted literally, but you can use an alternation to achieve the same effect:

(^|,)garp(,|$)

Demission

Nice, this is what I was looking for... Already looks a lot nicer than my crude or-chain... :) ty. – Contorted

keep in mind that in most situations ^ and $ match start and end of string, to make them match the start of the line use the m flag like so /^.*$/m – Hannon

Answer 2

you just need to use word boundary (\b) instead of ^ and $:

\bgarp\b

Bilbo

it's called word boundary, and while it'd work in this case it would fail if the value would have non-word characters in it, for example a dash: foo,garp-er,bar, it'd match garp even though there's no comma or end-of-string after it – Newbill

+1, beat me to it. Although in his example, he could simply use garp and it would work. Not really clear what his real requirements are... – Burnoose

@reko_t: thanks for the correction, it wouldn't fail though, because you'd be looking for garp-er. – Bilbo

@reko: Right. But if it gets this complicated, then a CSV parser might be an even better bet. Who knows if we might have commas embedded in strings? – Burnoose

@Tim: I suppose if another tag would be 'garper' then w/o word boundaries it would find both – Bilbo

@SilentGhost: yes, but my point was if you were to look for "garp". If you were to look for garp-er then similarly it'd fail for garp-er-er – Newbill

Lol. I needed this for VS Code's regex search and \b worked for me – Hogue

Answer 3

Just use look-arounds to solve this:

(?<=^|,)garp(?=$|,)

The difference with look-arounds and just regular groups are that with regular groups the comma would be part of the match, and with look-arounds it wouldn't. In this case it doesn't make a difference though.

Newbill

Alternatively you could group (garp) and get the result of that group though – Holmgren

@StephanBijzitter This doesn't work, because this would capture all words that contain garp as well, like abcgarp123. – Bendy

Answer 4

Without regex:

myString.Split(',').Contains("garp")

Kierakieran