CODEWITHSUNDEEP
regexbashgrep
darksky
darksky

Reputation: 21029

Matching strings with grep and \A regexp

Given the string in some file:

hel           string1
hell          string2
hello         string3

I'd like to capture just hel using cat file | grep 'regexp here'

I tried doing a bunch of regexp but none seem to work. What makes the most sense is: grep -E '\Ahel' but that doesn't seem to work. It works on http://rubular.com/ however. Any ideas why that isn't working with grep?

Also, when pasting the above string with a tab space before each line, the \A does not seem to work on rubular. I thought \A matches beginning of string, and that doesn't matter whatever characters was before that. Why did \A stop matching when there was a space before the string?

Upvotes: 3

Views: 395

Answers (4)

Jotne
Jotne

Reputation: 41450

Using awk

awk '$1=="hel"' file

PS you do not need to cat file to grep, use grep 'regexp here' file

Upvotes: 0

Mike Clark
Mike Clark

Reputation: 10136

ERE (-E) does not support \A for indicating start of match. Try ^ instead.

Use -m 1 to stop grepping after the first match in each file.

If you want grep to print only the matched string (not the entire line), use -o.

Use -h if you want to suppress the printing of filenames in the grep output.

Example:

 grep -Eohm 1 "^hel" *.log

If you need to enforce only outputting if the search string is on the first line of the file, you could use head:

 head -qn 1 *.log | grep -Eoh "^hel"

Upvotes: 3

Ibrahim Najjar
Ibrahim Najjar

Reputation: 19423

I thought \A matches beginning of string, and that doesn't matter whatever characters was before that. Why did \A stop matching when there was a space before the string?

\A matches the very beginning of the text, it doesn't match the start-of-line when you have one or more lines in your text.

Anyway, grep doesn't support \A so you need to use ^ which by the way matches the start of each line in multi-line mode contrary to \A.

Upvotes: 1

anubhava
anubhava

Reputation: 785316

ERE doesn't support \A but PCRE does hence grep -P can be used with same regex (if available):

grep -P '\Ahel\b' file
hel           string1

Also important is to use word boundary \b to restrict matching hello

Alternatively in ERE you can use:

egrep '^hel\b' 
hel           string1

Upvotes: 2

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