How to match start of line white space only when a specific word is after

Question

I want to match white space at the beginning of the line but only the the next word is PATCH, PUT, POST or GET

    edit_user_registration GET     /users/edit(.:format)      registrations#edit
                           PATCH   /users(.:format)           registrations#update

In the case above only the second line white space. I tried this:

(^\s*)+(POST|GET|DELETE|PUT|PATCH|OPTIONS)

Problem is this will match even the "PATCH" word but want to match only the whitespace before "PATCH".

Wiktor Stribiżew · Accepted Answer · 2016-04-07T14:11:00.453

2

You can use a lookahead instead:

^\s*(?=(?:POST|GET|DELETE|PUT|PATCH|OPTIONS)\b)

See the regex demo

The last \b makes the pattern match the words only as whole words. If it is not what you need, you can remove it.

A positive lookahead just checks if a pattern matches the text and returns true or false, either fail or pass, without consuming the characters.

Also, (\s*)+ (one or more zero or more whitespace symbols) can be shortened to just \s* (0+ whitespace symbols).

edited Apr 07 '16 at 14:11

answered Apr 07 '16 at 14:03

Wiktor Stribiżew

607,720
39
448
563

@Tushar: I believe it is not that necessary in this scenario, but I always prefer to play it safe. – Wiktor Stribiżew Apr 07 '16 at 14:07
Works with \b or without it. – code-gijoe Apr 07 '16 at 14:09
1

Shouldn't the `\b` be inside the lookahead? – Sebastian Proske Apr 07 '16 at 14:09
@SebastianProske: Yeah, what a typo - I must be rather tired now. – Wiktor Stribiżew Apr 07 '16 at 14:11
You might also consider using a possesive modifier or an atomic group for the `\s*`, to avoid unnecessary backtracking if there is no match. Not sure which regex flavour OP is using, so they might not be supported. – Sebastian Proske Apr 07 '16 at 14:23
Yes. The fact we do not know the regex flavor is always disturbing. Surely a possessive quantifier would be great to use here `^\s*+(?=(?:POST|GET|DELETE|PUT|PATCH|OPTIONS)\b)` – Wiktor Stribiżew Apr 07 '16 at 14:25

score 0 · Answer 2 · answered Apr 07 '16 at 14:03

0

You can use this :

^\s*(?=POST|GET|DELETE|PUT|PATCH|OPTIONS)

Try it here.

This regex will match the empty string on lines that start by the matched verb. If you don't want this behaviour, change the * to + :

^\s+(?=POST|GET|DELETE|PUT|PATCH|OPTIONS)

answered Apr 07 '16 at 14:03

Aaron

24,009
2
33
57

How to match start of line white space only when a specific word is after

2 Answers2