"unexpected numeric constant" when using mo() and $ factor level in interaction

[shirdekel]

I'd like to run a model with a monotonic predictor and an interaction with a variable that includes a dollar sign ("$") in the factor levels. I run into the following error:

suppressMessages({
  library(brms)
})

data <-
  data.frame(
    x = factor(letters[1:5], ordered = T),
    y = rnorm(1000),
    z = factor(c("a$", "b$"))
  )

## Works

brm(
  y ~ x * z,
  data = data,
  silent = 2,
  refresh = 0
)
#>  Family: gaussian 
#>   Links: mu = identity; sigma = identity 
#> Formula: y ~ x * z 
#>    Data: data (Number of observations: 1000) 
#>   Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
#>          total post-warmup draws = 4000
#> 
#> Regression Coefficients:
#>           Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> Intercept    -0.01      0.04    -0.10     0.08 1.00     4838     2738
#> x.L          -0.01      0.10    -0.20     0.19 1.00     3275     2768
#> x.Q           0.10      0.10    -0.10     0.29 1.00     3285     2791
#> x.C           0.02      0.10    -0.17     0.22 1.00     4100     2781
#> xE4          -0.12      0.10    -0.32     0.08 1.00     3585     2566
#> zb$          -0.00      0.07    -0.13     0.12 1.00     4948     2839
#> x.L:zb$       0.10      0.14    -0.18     0.37 1.00     3760     2753
#> x.Q:zb$      -0.06      0.14    -0.34     0.21 1.00     3259     2799
#> x.C:zb$      -0.06      0.14    -0.34     0.22 1.00     4179     3195
#> xE4:zb$       0.25      0.14    -0.03     0.52 1.00     3824     2900
#> 
#> Further Distributional Parameters:
#>       Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
#> sigma     0.99      0.02     0.95     1.04 1.00     5001     3102
#> 
#> Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
#> and Tail_ESS are effective sample size measures, and Rhat is the potential
#> scale reduction factor on split chains (at convergence, Rhat = 1).

## Doesn't work

brm(
  y ~ mo(x) * z,
  data = data,
  silent = 2,
  refresh = 0
)
#> Error in parse(text = x, keep.source = FALSE): <text>:2:0: unexpected end of input
#> 1: zb$
#>    ^

^{Created on 2025-03-14 with reprex v2.1.1}

Session info

sessioninfo::session_info()
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#>  date     2025-03-14
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[paul-buerkner]

I have checked the code and I don't think I can fix it in reasonable time. I may add an informative error message before the next release.