Why does grep -x imply parenthesizing the pattern?
While answering another question, I quoted man grep, in relation to -x
-x, --line-regexp
Select only those matches that exactly match the whole line.
For a regular expression pattern, this is like parenthesizing
the pattern and then surrounding it with ^ and $.
I already knew that -x was like "surrounding [the regex] with ^ and $", but why does it also imply "parenthesizing the pattern"?
I can't think of why parenthesizing would be necessary, or even change anything. If the whole pattern were parenthesized, you couldn't refer to it internally with a capturing group. grep complains about trailing backslashes, so there's no odd behaviour from an appended ) either (nor would I expect that to be in the spirit of an option).
Why does man grep specifically mention parenthesizing the pattern for -x?
grep
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
add a comment |
While answering another question, I quoted man grep, in relation to -x
-x, --line-regexp
Select only those matches that exactly match the whole line.
For a regular expression pattern, this is like parenthesizing
the pattern and then surrounding it with ^ and $.
I already knew that -x was like "surrounding [the regex] with ^ and $", but why does it also imply "parenthesizing the pattern"?
I can't think of why parenthesizing would be necessary, or even change anything. If the whole pattern were parenthesized, you couldn't refer to it internally with a capturing group. grep complains about trailing backslashes, so there's no odd behaviour from an appended ) either (nor would I expect that to be in the spirit of an option).
Why does man grep specifically mention parenthesizing the pattern for -x?
grep
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
add a comment |
While answering another question, I quoted man grep, in relation to -x
-x, --line-regexp
Select only those matches that exactly match the whole line.
For a regular expression pattern, this is like parenthesizing
the pattern and then surrounding it with ^ and $.
I already knew that -x was like "surrounding [the regex] with ^ and $", but why does it also imply "parenthesizing the pattern"?
I can't think of why parenthesizing would be necessary, or even change anything. If the whole pattern were parenthesized, you couldn't refer to it internally with a capturing group. grep complains about trailing backslashes, so there's no odd behaviour from an appended ) either (nor would I expect that to be in the spirit of an option).
Why does man grep specifically mention parenthesizing the pattern for -x?
grep
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
While answering another question, I quoted man grep, in relation to -x
-x, --line-regexp
Select only those matches that exactly match the whole line.
For a regular expression pattern, this is like parenthesizing
the pattern and then surrounding it with ^ and $.
I already knew that -x was like "surrounding [the regex] with ^ and $", but why does it also imply "parenthesizing the pattern"?
I can't think of why parenthesizing would be necessary, or even change anything. If the whole pattern were parenthesized, you couldn't refer to it internally with a capturing group. grep complains about trailing backslashes, so there's no odd behaviour from an appended ) either (nor would I expect that to be in the spirit of an option).
Why does man grep specifically mention parenthesizing the pattern for -x?
grep
grep
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
asked Feb 14 at 10:14
SparhawkSparhawk
10.1k64397
10.1k64397
add a comment |
add a comment |
1 Answer
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One instance where it’s important is for regular expressions with alternatives:
grep -E 'this|that'
If you only add ^ and $ without parenthesizing, this becomes
grep -E '^this|that$'
which matches lines starting with “this” or ending with “that”, rather than lines containing only “this” or “that”.
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
add a comment |
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1 Answer
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1 Answer
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One instance where it’s important is for regular expressions with alternatives:
grep -E 'this|that'
If you only add ^ and $ without parenthesizing, this becomes
grep -E '^this|that$'
which matches lines starting with “this” or ending with “that”, rather than lines containing only “this” or “that”.
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
add a comment |
One instance where it’s important is for regular expressions with alternatives:
grep -E 'this|that'
If you only add ^ and $ without parenthesizing, this becomes
grep -E '^this|that$'
which matches lines starting with “this” or ending with “that”, rather than lines containing only “this” or “that”.
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
add a comment |
One instance where it’s important is for regular expressions with alternatives:
grep -E 'this|that'
If you only add ^ and $ without parenthesizing, this becomes
grep -E '^this|that$'
which matches lines starting with “this” or ending with “that”, rather than lines containing only “this” or “that”.
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
One instance where it’s important is for regular expressions with alternatives:
grep -E 'this|that'
If you only add ^ and $ without parenthesizing, this becomes
grep -E '^this|that$'
which matches lines starting with “this” or ending with “that”, rather than lines containing only “this” or “that”.
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
answered Feb 14 at 10:17
Stephen KittStephen Kitt
174k24398473
174k24398473
add a comment |
add a comment |
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