Regex for "might" begin with, but "must" follow with

Question

I know this must be super simple but ...trying to match 2 use cases:

it can match this (exact)

var re = new RegExp("\\b" + name + "\\b");

or match this (same as above but MUST start with a space)

var re = new RegExp("^ \\b" + name + "\\b");

Actually the problem is that the string can contain multiple entries seperated by spaces like this

" somevar1 somevar2 somevar3 "

So when we pass name to the regex above we want it to match either at the beginning of the string with a space or at the beginning without space ..when we are not matching the beginning of the string (the rest), we do not test for a leading space

basically this "^ \\b" + name + "\\b|\\b" + name + "\\b"

Can you provide some examples of what you want to match (without mentioning any regex) — MxLDevs, Nov 12 '13 at 21:56
Please add examples of what you match. We can't help you if you don't. — OlivierH, Nov 12 '13 at 21:58
@Robert I still think you are overcomplicating this, as \b matches what you want. See http://stackoverflow.com/questions/1324676/what-is-a-word-boundary-in-regexes/1324784#1324784. Also if you trim your input string you won't have any whitespace at the start for sure. — CompuChip, Nov 13 '13 at 07:25

score 1 · Answer 1 · answered Nov 12 '13 at 21:57

1

Not sure why your first regex does not work for both cases. But I guess this is what you are looking for:

"(^ )?\\b" + name + "\\b"

answered Nov 12 '13 at 21:57

ahoereth

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Why do you put `^ ` inside parenthesis? – Casimir et Hippolyte Nov 12 '13 at 22:04
Yes, the white space is in my comment but it's not very visible. – Casimir et Hippolyte Nov 12 '13 at 22:40
Ah sorry. Wrapped both in paranthesis to make them optional together. Might want to change it to `(?:^ )?` to hide it from the result. – ahoereth Nov 14 '13 at 22:38
No, it's me, I haven't understood your idea, but now it's ok. Indeed you could use non-capturing group, but it isn't essential. – Casimir et Hippolyte Nov 14 '13 at 22:45

Casimir et Hippolyte · Accepted Answer · 2013-11-12T22:52:14.583

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Ok, you can use this pattern:

new RegExp("(?:^ |\\b)" + name + "\\b");

details:

(?:       # non capturing group
    ^[ ]  # start of the string followed by a space
  |       # OR
    \\b   # word boundary
)         # close the non capturing group

Here you have the choice between the start of the string with a space and a word boundary (that can be, for example .name or #name or name)

with " ?\\b" + name, since the space is optional, you can have name at the start of the string.

edited Nov 12 '13 at 22:52

answered Nov 12 '13 at 22:09

Casimir et Hippolyte

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Thanks but i got my code mixed up ! :) What works for my particular code is actually ``" ?\\b" + name + "\\b"`` but ``"\s?\\b" + name + "\\b"`` does not ..what's the difference ? i thought \s is space ? – Robert Hoffmann Nov 12 '13 at 22:25
thanks again for the details. however isn't ``"\s?\\b" + name + "\\b"`` the same as ``" ?\\b" + name + "\\b"``. I'd say yes, but in my code they both behave differently – Robert Hoffmann Nov 12 '13 at 23:05

score 0 · Answer 3 · answered Nov 12 '13 at 21:57

0

i guess you need something like this:

new RegExp("^ ?\\b" + name + "\\b");

or like this:

new RegExp("^ *\\b" + name + "\\b");

answered Nov 12 '13 at 21:57

micnic

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Regex for "might" begin with, but "must" follow with

3 Answers3