Regex to match only the first line?

Question

Is it possible to make a regex match only the first line of a text? So if I have the text:

This is the first line.
This is the second line. ...

It would matc

Answer 1

Yes, you can.

Example in javascript:

"This is the first line.\n This is the second line.".match(/^.*$/m)[0];

Returns

"This is the first line."

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EDIT

Explain regex:

match(/^.*$/m)[0]

• ^: begin of line
• .*: any char (.), 0 or more times (*)
• $: end of line.
• m: multiline mode (. acts like a \n too)
• [0]: get first position of array of results

Answer 2

There is also negative lookbehind function (PowerGREP or Perl flavor). It works perfectly for my purposes. Regex:

(?<!\s+)^(.+)$

where

• (?<!\s+) is negative lookbehind - regex matches only strings that are not preceded by a whitespace(s) (\s also stands for a line break)
• ^ is start of a string
• (.+) is a string
• $ is end of string

Answer 3

In case you need the very first line no matter what, here you go:

 \A.*

It will select the first line, no matter what.

Answer 4

that's sounds more like a job for the filehandle buffer.

You should be able to match the first line with:

/^(.*)$/m

(as always, this is PCRE syntax)

the /m modifier makes ^ and $ match embedded newlines. Since there's no /g modifier, it will just process the first occurrence, which is the first line, and then stop.

If you're using a shell, use:

head -n1 file

or as a filter:

commandmakingoutput | head -n1

Please clarify your question, in case this is not wat you're looking for.