Match start of line in multiline string in lua?

Question

Let's say I want to match any sequence of the hash sign # at the start of a string; so I'd want to match ## here:

local mystr = "##First line\nSecond line\nThird line"

... and ### here:

local mystr = "First line\nSecond line\n### Third line"

... and nothing here:

local mystr = "First line\nSecond line\nThird line"

I guess, the pattern ^(%#+) could have worked here if the caret ^ meant start of a (multiline) line - however, as far as I can see, e.g. in How to match the start or end of a string with string.match in Lua?

^ at the start of a pattern anchors it to the start of the string. $ at the end of a pattern anchors it to the end of the string.

... the caret here would only match at the start of the multiline string (mystr) itself.

How could I go about matching a pattern that starts with "line start" in Lua?

Mikael Öhman · Accepted Answer · 2025-05-07 22:24:55Z

3

You want to match either the start of the string ^ or after any newline \n character:

mystr:match("^(%#+)") or mystr:match("\n(%#+)")

I don't think you can combine it into one match. Logical 'or' in Lua patterns?

answered May 7 at 22:24

Mikael Öhman

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Heliodex · Accepted Answer · 2025-05-09 13:10:35Z

An option would be to just add a \n to the start of the string:

string.match("\n" .. s, "\n(#+)")

^ can only be used as the first character of a pattern, and [charsets] only allow for single characters rather than sets of groups, so I don't think there exists a pattern that will match a string of any length (even with multiple captures).

(Edit: I tried with frontier patterns but couldn't get it working, see tehtmi's answer for a pattern that works with a single capture.)

It's also possible with a loop and multiple string.match() calls. This function matches every sequence of #s at the start of a line:

local function match(s)
    local matches = {}
    for _, v in ipairs(string.split(s, "\n")) do
        local m = string.match(v, "^#+")
        if m then
            table.insert(matches, m)
        end
    end
    return matches
end

tehtmi · Accepted Answer · 2025-05-08 23:37:11Z

2

It's a little more esoteric, but (if you aren't worried about null characters / embedded zeroes in your string), you could use the frontier pattern to do it directly with one pattern and no additional processing, like "%f[^\0\n](%#+)"

The frontier pattern matches a position where the next character belongs to the indicated set and the previous character doesn't -- and outside the string is treated as the null character '\0' which is useful if your string doesn't otherwise use the null character. In this case, "%f[^\0\n]" matches a position at the start of the string or after a newline and followed by any other character (including '#').

answered May 8 at 23:37

tehtmi

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2 Comments

QuentinC May 9 at 3:37

It is worth to note that frontier pattern only exists since lua 5.2. Thus, in particular, it won't work with luajit.

tehtmi May 9 at 18:39

@QuenticC: Actually, it does exist in Lua 5.1 as an undocumented feature, but note that Lua 5.1 patterns aren't allowed to contain embedded zeroes, so you need to use %z instead of \0.

Monev · Accepted Answer · 2025-05-09 05:18:11Z

Be aware, # is not a special character, you do not need to escape it with %

The correct answer for your first question is lehtmi's, Using frontier pattern is the way to go, and yes it does work on latest 5.1 and luajit to make a match using string.match

string.match(str, "%f[^\n\r\0]###?[^#\r\n\0]+")

Should yield ##First line

How could I go about matching a pattern that starts with "line start" in Lua?

By matching everything that is not a line, you can divide using string.gmatch.

for line in str:gmatch"[^\r\n]*[^\n]?" do
 if line:find"^###?.+" then -- stuff here
 end
end

Should be enough for your use case.

A cool little trick is also using io.lines

local H = io.tmpfile()
H:write(str)
for line in H:lines() do
 if line:match then -- ... Same as above.
end
H:close()

This does sacrifice performance but it does allow for whatever io.lines does, which means it is less multiplatform.

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