# Transducer Filtering with Regex?

**URL:** https://discourse.julialang.org/t/transducer-filtering-with-regex/52441
**Category:** General Usage
**Tags:** strings, regex, lazy-evaluation
**Created:** [December 27, 2020, 4:26am UTC](https://discourse.julialang.org/t/transducer-filtering-with-regex/52441 "2020-12-27T04:26:39Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![Satvik](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/satvik/32/20486_2.png) [@Satvik](https://discourse.julialang.org/u/Satvik)
#### Post date: [December 27, 2020, 4:26am UTC](https://discourse.julialang.org/t/transducer-filtering-with-regex/52441/1 "2020-12-27T04:26:39Z")

</div>

Hi,

I have several cases where I need just the first few elements that match a condition. With transducers, I can do something like

```julia
big_array |> Filter(expensive_filter) |> Map(expensive_transformation) |> Take(5) |> collect

```

and it will evaluate my expensive functions the minimum number of times to get 5 passing elements. Awesome!

I’m trying to do something similar with regex. Here’s a working, non-transducer example:

```julia
function get_regex_matches(str, pattern, cache, num_matches)
    j = 1
    for i in 1:num_matches
        m = findnext(pattern, str, j)
        cache[i] = SubString(str, m.start, m.stop)
        j = m.stop
    end
end

```

I double-checked that this approach is actually faster than using `eachmatch` for my use case:

```julia
function get_regex_matches_eachmatch(str, pattern, cache, num_matches)
    for (i, m) in enumerate(eachmatch(pattern, str))
        if i > num_matches
            break
        end
        cache[i] = m.match
    end
end

```

```julia
sample_string = "np.where(0.03376436965056531*df['rolling_zscore(sma(high_low_mean,12),90)']+ 0.05938041870616624*df['rolling_zscore(slope(slope(slope(24),24),12),90)'] - 0.05545017417059229*df['rolling_zscore(slope(slope(slope(12),12),12),90)'] + -0.02641195019844662 > 0, 1.0, "
cache = Array{String}(undef, 4)
const feature_pat = Regex(raw"\[.*?\]")

@btime get_regex_matches_eachmatch(sample_string, feature_pat, cache, 2)
  1.085 μs (13 allocations: 1.11 KiB)
id = x -> x
@btime get_regex_matches(sample_string, feature_pat, cache, id, 2)
  617.279 ns (5 allocations: 240 bytes)

```

However, this loses the benefits of the transducer approach, especially composability. Is there a way to do a transducer-like or lazy filter that goes through regex matches of a string?