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Regexp Backslash - GNU Emacs Manual says that \< matches at the beginning of a word, \> matches at the end of a word, and \b matches a word boundary. \b is just as in other non-Emacs regular expressions. But it seems that \< and \> are particular to Emacs regular expressions. Are there cases where \< and \> are needed instead of \b? For instance, \bword\b would match the same as \<word\> would, and the only difference is that the latter is more readable.

Yoo
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2 Answers2

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You can get unexpected results if you assume they behave the same..
What can \< and > that \b can do?
The answer is that \< and\> are explicit... This end of a word! and only this end!
\bis general.... Either end of a word will match...

GNU Operators * Word Operators

line="cat dog sky"  
echo "$line" |sed -n "s/\(.*\)\b\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\>\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\<\(.*\)/# |\1|\2|/p"
echo
line="cat  dog  sky"  
echo "$line" |sed -n "s/\(.*\)\b\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\>\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\<\(.*\)/# |\1|\2|/p"
echo
line="cat  dog  sky  "  
echo "$line" |sed -n "s/\(.*\)\b\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\>\(.*\)/# |\1|\2|/p"
echo "$line" |sed -n "s/\(.*\)\<\(.*\)/# |\1|\2|/p"
echo

output 

# |cat dog |sky|
# |cat dog| sky|
# |cat dog |sky|

# |cat  dog  |sky|
# |cat  dog|  sky|
# |cat  dog  |sky|

# |cat  dog  sky|  |
# |cat  dog  sky|  |
# |cat  dog  |sky  |
Peter.O
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2

It looks to me like \<.*?\> would match only series of word characters, while \b.*?\b would match either series of word characters or a series non-word characters, since it can also accept the end of a word, and then the beginning of one. If you force the expression between the two to be a word, they do indeed act the same.

Of course, you could replicate the behavior of \< and \> with \b\w and \w\b. So I guess the answer is that yes, it's mostly for readability. Then again, isn't that what most escape characters in regular expression are for?

dlras2
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  • The Escape char `\\` is never for readability. It is used to differentiate a *regex operator* from a *literal* character of the same glyph – Peter.O Apr 30 '11 at 23:35
  • @fred - What I meant was that the escaped characters such as `\w` and `\d` (not `\ ` itself) can usually be replaced with other characters of a character class, like `[0-9]`. – dlras2 May 01 '11 at 03:13
  • Daniel: `\<.*\>` will match any string bounded by word characters. The `.*` is greedy, so matches as many arbitrary characters as possible. To match only individual words, you could use a non-greedy variant: `\<.*?\>` – phils May 01 '11 at 09:14