Quoting regular expressions in Bash

Question

I'm trying to write a condition that would fit all the lines starting with a space/tab and the word Path

/sPath.* - simple regexp?

I found that in Bash 4.* it should look like:

if [[ $LINE =~ "[[:space:]]Path" ]]

But this condition for some reason does not work.

if [[ $LINE =~ [[:space:]] ]]

work fine, and displays all lines with spaces/tabs.

Roberto Reale · Answer 1 · 2014-05-12T08:50:14.107

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From version 3.2 onwards, the pattern (i.e., the regular expression) must not be quoted in Bash:

New Features in Bash

...

f. Quoting the string argument to the [[ command's =~ operator now forces string matching, as with the other pattern-matching operators.

In other words, quoting is considered part of the regular expression itself (literal ").

Moreover, it would be better to quote the variable $LINE, to prevent errors should it be empty:

if [[ "$LINE" =~ [[:space:]] ]]

edited May 12 '14 at 08:50

answered May 12 '14 at 08:44

Roberto Reale

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What errors can happen if $LINE is not quoted in `[[ ... ]]`? – choroba May 12 '14 at 08:57
And if i match with string [[:blank:]]Path* where * - any symbols? – aranel May 12 '14 at 08:59

score 0 · Accepted Answer · answered May 12 '14 at 08:46

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It is better to use:

[[ "$LINE" =~ [[:blank:]] ]]

Quote the variable LINE
Match with character class [[:blank:]] which is equivalent of space OR tab

answered May 12 '14 at 08:46

anubhava

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Why is it better to quote the variable? – choroba May 12 '14 at 08:56
And if i match with string [[:blank:]]Path* where * - any symbols? – aranel May 12 '14 at 08:58
You can use: `[[ "$LINE" =~ [[:blank:]]"Path" ]]` – anubhava May 12 '14 at 08:59
@choroba: It is because `$LINE` contains spaces at start – anubhava May 12 '14 at 09:00
@anubhava: So what? `l=' a'; [[ $l =~ ^' ' ]] && echo Matches` – choroba May 12 '14 at 09:03
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@choroba: quoting may be useful because of field splitting (tokens), e.g. `[[ a␣␣␣b =~ "a␣␣␣b" ]]` (replace `␣` with a space). – Dec 23 '18 at 16:19
Works without quotes: `line='a␣␣␣b'; [[ $line =~ 'a␣␣␣b' ]]` – choroba Dec 23 '18 at 22:19

Quoting regular expressions in Bash

2 Answers2